amphtml-validator: Add --user-agent option and default user agent. (#5115)
amphtml-validator: Add --user-agent option and default user agent. (#5115)

* amphtml-validator: Add --user-agent option and default user agent.

* amphtml-validator: Removing version from user agent, as asked in pull-request.

widespace-os/amphtml,aghassemi/amphtml,rlugojr/amphtml,dvoytenko/amphtml,BenDz/amphtml,nexxtv/amphtml,engtat/amphtml,clawr/amphtml,rsimha-amp/amphtml,triplelift/amphtml,rifler/amphtml,mlb7687/amphtml,zhouyx/amphtml,ecoron/amphtml,ericlindley-g/amphtml,lzanol/amphtml,rajkumarsrk/amphtml,jlucero-contentad/amphtml,wjfang/amphtml,jlucero-contentad/amphtml,jpettitt/amphtml,rifler/amphtml,arealit/amphtml,lzanol/amphtml,mlb7687/amphtml,burtcorp/amphtml,sebastianbenz/amphtml,honeybadgerdontcare/amphtml,shawnbuso/amphtml,aghassemi/amphtml,src-code/amphtml,yieldmo/amphtml,donttrustthisbot/amphtml,pietergreyling/amphtml,spotxchange/amphtml,voyagegroup/amphtml,rlugojr/amphtml,mhc-gh/amphtml,zhangsu/amphtml,yandex-pcode/amphtml,adup-tech/amphtml,aghassemi/amphtml,jlucero-contentad/amphtml,interspacejp/amphtml,Lith/amphtml,ericlindley-g/amphtml,widespace-os/amphtml,DistroScale/amphtml,avimehta/amphtml,taboola/amphtml,prateekbh/amphtml,rsimha-amp/amphtml,src-code/amphtml,DarXector/amphtml,media-net/amphtml,lannka/amphtml,ampproject/amphtml,wjfang/amphtml,ruturajv-media-net/amphtml,sriramkrish85/amphtml,yieldbot/amphtml,zhouyx/amphtml,dotandads/amphtml,arealit/amphtml,laterpay/amphtml,Gregable/amphtml,triplelift/amphtml,rajkumarsrk/amphtml,jmadler/amphtml,ampproject/amphtml,Adtoma/amphtml,sortable/amphtml,donttrustthisbot/amphtml,src-code/amphtml,JuliusThms/amphtml,ampproject/amphtml,BenDz/amphtml,camelburrito/amphtml,ampproject/amphtml,smartadserver/amphtml,Lith/amphtml,burtcorp/amphtml,Adtoma/amphtml,Gregable/amphtml,ericlindley-g/amphtml,yandex-pcode/amphtml,JuliusThms/amphtml,rajkumarsrk/amphtml,nexxtv/amphtml,smartadserver/amphtml,lzanol/amphtml,tlong2/amphtml,yandex-pcode/amphtml,DAC-f1h-dev/amphtml,yieldbot/amphtml,arealit/amphtml,sharethrough/amphtml,mhc-gh/amphtml,tdrl/amphtml,nexxtv/amphtml,zhucl/amphtml,rifler/amphtml,kamtschatka/amphtml,wjfang/amphtml,adup-tech/amphtml,jmadler/amphtml,laterpay/amphtml,engtat/amphtml,dvoytenko/amphtml,triplelift/amphtml,DistroScale/amphtml,luciancrasovan/amphtml,DarXector/amphtml,engtat/amphtml,kmh287/amphtml,tdrl/amphtml,sharethrough/amphtml,rajkumarsrk/amphtml,rlugojr/amphtml,Mobusi-ads/amphtml,sortable/amphtml,sebastianbenz/amphtml,jpettitt/amphtml,BenDz/amphtml,DAC-f1h-dev/amphtml,danielrozenberg/amphtml,wjfang/amphtml,devjsun/amphtml,taboola/amphtml,cory-work/amphtml,yandex-pcode/amphtml,gumgum/amphtml,shawnbuso/amphtml,luciancrasovan/amphtml,ruturajv-media-net/amphtml,jpettitt/amphtml,tdrl/amphtml,devjsun/amphtml,ericlindley-g/amphtml,devjsun/amphtml,nexxtv/amphtml,smartadserver/amphtml,keithwrightbos/amphtml,adtechnology/amphtml,taboola/amphtml,triplelift/amphtml,dotandads/amphtml,zhouyx/amphtml,Lith/amphtml,powdercloud/amphtml,Lith/amphtml,comScore/amphtml,sriramkrish85/amphtml,taboola/amphtml,sklobovskaya/amphtml,jdelhommeau/amphtml,sebastianbenz/amphtml,sortable/amphtml,Lith/amphtml,jmadler/amphtml,JuliusThms/amphtml,BrightcoveOS/amphtml,DistroScale/amphtml,cory-work/amphtml,shawnbuso/amphtml,widespace-os/amphtml,alanorozco/amphtml,avimehta/amphtml,comScore/amphtml,powdercloud/amphtml,donttrustthisbot/amphtml,bpaduch/amphtml,BrightcoveOS/amphtml,pietergreyling/amphtml,comScore/amphtml,comScore/amphtml,ecoron/amphtml,kixer-com/amphtml,laterpay/amphtml,jridgewell/amphtml,mlb7687/amphtml,ecoron/amphtml,jmadler/amphtml,jdelhommeau/amphtml,DistroScale/amphtml,engtat/amphtml,yieldbot/amphtml,tlong2/amphtml,engtat/amphtml,yieldmo/amphtml,rifler/amphtml,cvializ/amphtml,sebastianbenz/amphtml,sriramkrish85/amphtml,rsimha-amp/amphtml,jridgewell/amphtml,vmarteev/amphtml,bpaduch/amphtml,BenDz/amphtml,fullspeed-sp/amphtml,ampproject/amphtml,widespace-os/amphtml,rlugojr/amphtml,ampproject/amphtml,pietergreyling/amphtml,burtcorp/amphtml,dotandads/amphtml,interspacejp/amphtml,src-code/amphtml,vmarteev/amphtml,luciancrasovan/amphtml,jdelhommeau/amphtml,rlugojr/amphtml,dotandads/amphtml,ericlindley-g/amphtml,DAC-f1h-dev/amphtml,arealit/amphtml,voyagegroup/amphtml,clawr/amphtml,kamtschatka/amphtml,honeybadgerdontcare/amphtml,zhangsu/amphtml,yieldmo/amphtml,avimehta/amphtml,techhtml/amphtml,nexxtv/amphtml,adiant/amphtml,media-net/amphtml,jridgewell/amphtml,gumgum/amphtml,sklobovskaya/amphtml,tlong2/amphtml,jlucero-contentad/amphtml,jdelhommeau/amphtml,kamtschatka/amphtml,yandex-pcode/amphtml,devjsun/amphtml,kmh287/amphtml,tlong2/amphtml,vmarteev/amphtml,rifler/amphtml,sklobovskaya/amphtml,sklobovskaya/amphtml,arealit/amphtml,camelburrito/amphtml,zhouyx/amphtml,media-net/amphtml,shawnbuso/amphtml,sharethrough/amphtml,shawnbuso/amphtml,lannka/amphtml,ampproject/amphtml,nexxtv/amphtml,wjfang/amphtml,jpettitt/amphtml,pietergreyling/amphtml,interspacejp/amphtml,zhucl/amphtml,rsimha-amp/amphtml,adup-tech/amphtml,pietergreyling/amphtml,media-net/amphtml,fullspeed-sp/amphtml,cvializ/amphtml,sebastianbenz/amphtml,erwinmombay/amphtml,Mobusi-ads/amphtml,adtechnology/amphtml,dvoytenko/amphtml,Gregable/amphtml,sklobovskaya/amphtml,honeybadgerdontcare/amphtml,honeybadgerdontcare/amphtml,alanorozco/amphtml,mhc-gh/amphtml,adiant/amphtml,tdrl/amphtml,JuliusThms/amphtml,erwinmombay/amphtml,ruturajv-media-net/amphtml,engtat/amphtml,ecoron/amphtml,lzanol/amphtml,BrightcoveOS/amphtml,avimehta/amphtml,arealit/amphtml,kmh287/amphtml,DAC-f1h-dev/amphtml,mhc-gh/amphtml,yandex-pcode/amphtml,DistroScale/amphtml,Gregable/amphtml,cvializ/amphtml,prateekbh/amphtml,taboola/amphtml,aghassemi/amphtml,media-net/amphtml,gumgum/amphtml,jpettitt/amphtml,danielrozenberg/amphtml,voyagegroup/amphtml,sharethrough/amphtml,techhtml/amphtml,tlong2/amphtml,fullspeed-sp/amphtml,pietergreyling/amphtml,jridgewell/amphtml,pedjoni8/amphtml,kmh287/amphtml,koshka/amphtml,gumgum/amphtml,taboola/amphtml,sriramkrish85/amphtml,pedjoni8/amphtml,pedjoni8/amphtml,clawr/amphtml,kamtschatka/amphtml,zhucl/amphtml,zhucl/amphtml,yieldbot/amphtml,prateekbh/amphtml,Gregable/amphtml,clawr/amphtml,pedjoni8/amphtml,rsimha-amp/amphtml,interspacejp/amphtml,techhtml/amphtml,erwinmombay/amphtml,rajkumarsrk/amphtml,Adtoma/amphtml,koshka/amphtml,danielrozenberg/amphtml,zhangsu/amphtml,honeybadgerdontcare/amphtml,zhucl/amphtml,jdelhommeau/amphtml,kixer-com/amphtml,spotxchange/amphtml,smartadserver/amphtml,techhtml/amphtml,aghassemi/amphtml,alanorozco/amphtml,rsimha-amp/amphtml,widespace-os/amphtml,erwinmombay/amphtml,danielrozenberg/amphtml,mhc-gh/amphtml,ruturajv-media-net/amphtml,BrightcoveOS/amphtml,nexxtv/amphtml,voyagegroup/amphtml,comScore/amphtml,burtcorp/amphtml,bpaduch/amphtml,cory-work/amphtml,Adtoma/amphtml,mhc-gh/amphtml,chaveznvg/amphtml,sriramkrish85/amphtml,clawr/amphtml,kmh287/amphtml,yieldbot/amphtml,zhucl/amphtml,alanorozco/amphtml,BrightcoveOS/amphtml,JuliusThms/amphtml,fullspeed-sp/amphtml,kamtschatka/amphtml,camelburrito/amphtml,dvoytenko/amphtml,sortable/amphtml,devjsun/amphtml,ruturajv-media-net/amphtml,bpaduch/amphtml,DistroScale/amphtml,ADITION/amphtml,widespace-os/amphtml,gumgum/amphtml,powdercloud/amphtml,keithwrightbos/amphtml,laterpay/amphtml,cory-work/amphtml,chaveznvg/amphtml,chaveznvg/amphtml,Adtoma/amphtml,jmadler/amphtml,laterpay/amphtml,keithwrightbos/amphtml,engtat/amphtml,adiant/amphtml,ecoron/amphtml,jmadler/amphtml,voyagegroup/amphtml,kixer-com/amphtml,shawnbuso/amphtml,src-code/amphtml,powdercloud/amphtml,powdercloud/amphtml,honeybadgerdontcare/amphtml,dvoytenko/amphtml,ADITION/amphtml,rajkumarsrk/amphtml,adup-tech/amphtml,wjfang/amphtml,media-net/amphtml,mhc-gh/amphtml,chaveznvg/amphtml,jpettitt/amphtml,luciancrasovan/amphtml,zhangsu/amphtml,clawr/amphtml,alanorozco/amphtml,jpettitt/amphtml,zhouyx/amphtml,laterpay/amphtml,donttrustthisbot/amphtml,donttrustthisbot/amphtml,voyagegroup/amphtml,lannka/amphtml,media-net/amphtml,luciancrasovan/amphtml,vmarteev/amphtml,alanorozco/amphtml,mlb7687/amphtml,prateekbh/amphtml,fullspeed-sp/amphtml,ericlindley-g/amphtml,devjsun/amphtml,danielrozenberg/amphtml,mhc-gh/amphtml,erwinmombay/amphtml,koshka/amphtml,vmarteev/amphtml,comScore/amphtml,powdercloud/amphtml,donttrustthisbot/amphtml,camelburrito/amphtml,comScore/amphtml,jlucero-contentad/amphtml,gumgum/amphtml,bpaduch/amphtml,mlb7687/amphtml,gumgum/amphtml,sriramkrish85/amphtml,JuliusThms/amphtml,yieldbot/amphtml,vmarteev/amphtml,interspacejp/amphtml,honeybadgerdontcare/amphtml,adtechnology/amphtml,DarXector/amphtml,koshka/amphtml,adup-tech/amphtml,camelburrito/amphtml,voyagegroup/amphtml,adtechnology/amphtml,danielrozenberg/amphtml,cory-work/amphtml,techhtml/amphtml,prateekbh/amphtml,jpettitt/amphtml,ecoron/amphtml,Mobusi-ads/amphtml,dotandads/amphtml,tdrl/amphtml,dvoytenko/amphtml,kixer-com/amphtml,clawr/amphtml,yieldmo/amphtml,cvializ/amphtml,ADITION/amphtml,DAC-f1h-dev/amphtml,luciancrasovan/amphtml,zhucl/amphtml,bpaduch/amphtml,taboola/amphtml,yandex-pcode/amphtml,spotxchange/amphtml,kixer-com/amphtml,jridgewell/amphtml,lzanol/amphtml,triplelift/amphtml,powdercloud/amphtml,sortable/amphtml,lannka/amphtml,vmarteev/amphtml,alanorozco/amphtml,rsimha-amp/amphtml,BenDz/amphtml,keithwrightbos/amphtml,shawnbuso/amphtml,cvializ/amphtml,wjfang/amphtml,prateekbh/amphtml,chaveznvg/amphtml,danielrozenberg/amphtml,cory-work/amphtml,kixer-com/amphtml,koshka/amphtml,rajkumarsrk/amphtml,dotandads/amphtml,DarXector/amphtml,pedjoni8/amphtml,yieldmo/amphtml,rlugojr/amphtml,adtechnology/amphtml,spotxchange/amphtml,Gregable/amphtml,danielrozenberg/amphtml,techhtml/amphtml,prateekbh/amphtml,sebastianbenz/amphtml,alanorozco/amphtml,ampproject/amphtml,bpaduch/amphtml,mlb7687/amphtml,Adtoma/amphtml,avimehta/amphtml,tlong2/amphtml,media-net/amphtml,adiant/amphtml,src-code/amphtml,jlucero-contentad/amphtml,camelburrito/amphtml,lzanol/amphtml,fullspeed-sp/amphtml,jridgewell/amphtml,zhangsu/amphtml,yieldmo/amphtml,yieldmo/amphtml,sortable/amphtml,rifler/amphtml,Lith/amphtml,jridgewell/amphtml,DarXector/amphtml,BrightcoveOS/amphtml,kixer-com/amphtml,adtechnology/amphtml,donttrustthisbot/amphtml,smartadserver/amphtml,cory-work/amphtml,kamtschatka/amphtml,Mobusi-ads/amphtml,zhouyx/amphtml,ruturajv-media-net/amphtml,avimehta/amphtml,erwinmombay/amphtml,smartadserver/amphtml,Mobusi-ads/amphtml,sharethrough/amphtml,Gregable/amphtml,ADITION/amphtml,sharethrough/amphtml,DAC-f1h-dev/amphtml,burtcorp/amphtml,adiant/amphtml,pedjoni8/amphtml,keithwrightbos/amphtml,spotxchange/amphtml,zhangsu/amphtml,lannka/amphtml,triplelift/amphtml,aghassemi/amphtml,ADITION/amphtml,BenDz/amphtml,zhouyx/amphtml,arealit/amphtml,keithwrightbos/amphtml,sklobovskaya/amphtml,adiant/amphtml,chaveznvg/amphtml,interspacejp/amphtml,kmh287/amphtml,Gregable/amphtml,lannka/amphtml,camelburrito/amphtml,keithwrightbos/amphtml,burtcorp/amphtml,Mobusi-ads/amphtml,sklobovskaya/amphtml,spotxchange/amphtml,lannka/amphtml,cvializ/amphtml,luciancrasovan/amphtml,adiant/amphtml,DarXector/amphtml,tdrl/amphtml,adup-tech/amphtml,jdelhommeau/amphtml,koshka/amphtml