remove duplicate code for tags and categories
remove duplicate code for tags and categories

Previously the `Site#tags` and `Site#categories` actually had the
same structure as a hash returned by `Site#post_attr_hash()`, but
maintained separately. Remove this duplicated infrastructure. For
backwards-compatibility for plugins, provide aliases for them.

noikiy/jekyll,tareq-s/jekyll,adilapapaya/jekyll,SQS2/Test,xiaoshaozi52/jekyll,saitodisse/jekyll,marrujoalex/jekyll,Wirachmat/jekyll,fulldecent/jekyll,naughtyboy83/jekyll,gynter/jekyll,iRoxx/jekyll,fengsmith/jekyllcn,goragod/jekyll,MjAbuz/jekyll,Kinghack/kinghack.github.com,xiongjungit/jekyll,honger05/jekyllcn,ZainRizvi/jekyll,ReachingOut/jekyll,fabulousu/jekyll,angeliaz/jekyll,cyberid41/jekyll,floydpraveen/jekyll,daniel-beck/jekyll,winndows/jekyll,monxim/monxim.github.io,greent2008/jekyll,CPVPLachlan/CPVP-Blog,ewinkler/jekyll,fulldecent/jekyll,darwin/jekyll,harrissoerja/jekyll,krahman/jekyll,zhangkuaiji/jekyllcn,sideci-sample/sideci-sample-jekyll,sideci-sample/sideci-sample-jekyll,getandpost/jekyll.github.io,xiebinhqy/jklly_china,oresmus/jekyll,CPVPLachlan/CPVP-Blog,ducktyper/jekyll,gaosboy/jekyll,gencer/jekyll,nemotan/jekyll,imaustink/jekyll,rasa2011/rasa2011.github.io,Teino1978-Corp/Teino1978-Corp-jekyll,CorainChicago/jekyll,xtymichael/jekyll,vcgato29/jekyll,vcgato29/jekyll,fengsmith/jekyllcn,teju111/jekyll,jekyll/jekyll,kimeng/jekyll,martynbm/jekyll,kimeng/jekyll,martynbm/jekyll,x-way/jekyll,angeliaz/jekyll,Tyrion22/jekyll,tillgrallert/jekyll,digideskio/jekyll,garvitr/jekyll,pheuko/jekyll,CheekyFE/jekyll,pheuko/jekyll,hartmel/jekyll,chrisfinazzo/jekyll,doubleday/jekyll,chiyodad/jekyll,AaronSikes/jekyll,codeclimate-testing/jekyll,backendeveloper/jekyll,dezon/jekyll,vidbina/jekyll,ls2uper/jekyll,wadaries/jekyll,getandpost/jekyll.github.io,brint/jekyll,thetaxman/jekyll,rasa2011/rasa2011.github.io,fulldecent/jekyll,ZainRizvi/jekyll,tomasdiez/jekyll,kinlane/jekyll,xantage/jekyll,fabulousu/jekyll,sanxore/sanxore.github.com,m2candre/jekyll,aarvay/jekyll,yihangho/jekyll,Tiger66639/jekyll,chinayin/jekyll,tomasdiez/jekyll,gencer/jekyll,tugberkugurlu/jekyll,honger05/jekyllcn,zhangkuaiji/jekyllcn,jmepg/jekyll,mixxmac/jekyll,jmhardison/jekyll,hartmel/jekyll,trungvothanh/jekyll,floydpraveen/jekyll,doubleday/jekyll,tobscure/jekyll,xiebinhqy/jklly_china,x-way/jekyll,floydpraveen/jekyll,stomar/jekyll,johnnycastilho/jekyll,chrisfinazzo/jekyll,LuoPX-15510486969/jekyll,htmelvis/jekyll,LeuisKen/jekyllcn,eloyesp/jekyll,krishnakalyan3/jekyll,nathanhorrigan1/webdeveloper,ZDroid/jekyll,tylermachen/jekyll,18F/jekyll,richsoni/jekyll,drobati/jekyll,EddieDow/jekyll,LuoPX-15510486969/jekyll,mnuessler/jekyll,anthonyrosengren/grasshoppergathering2.github-io,eninya/jekyll,nasht00/jekyll,rajrathore/jekyll,xcatliu/jekyllcn,princeofdarkness76/jekyll,gkunwar/gkunwar.github.io,ewinkler/jekyll,gencer/jekyll,gencer/jekyll,stomar/jekyll,liukaijv/jekyll,marrujoalex/jekyll,gkunwar/gkunwar.github.io,CPVPLachlan/CPVP-Blog,aarvay/jekyll,hartmel/jekyll,saitodisse/jekyll,SQS2/Test,ajhit406/jekyll,wadaries/jekyll,tareq-s/jekyll,gencer/jekyll,ajhit406/jekyll,egobrightan/jekyll,jmknoll/jekyll,alex-kovac/jekyll,pheuko/jekyll,hiteshsuthar/jekyll,nemotan/jekyll,Wirachmat/jekyll,desidude03/jekyll,CorainChicago/jekyll,Wirachmat/jekyll,derekgottlieb/jekyll,MauroMrod/jekyll,vcgato29/jekyll,CorainChicago/jekyll,shuber2/jekyll,rovrevik/jekyll,PepSalehi/jekyll,LeuisKen/jekyllcn,thejameskyle/jekyll,abhilashhb/jekyll,fengsmith/jekyll,akoeplinger/jekyll,jmptrader/jekyll,jekyll/jekyll,omeripek/jekyll,indraj/jekyll,Strangehill/jekyll,drobati/jekyll,ramoslin02/jekyll,desidude03/jekyll,alihalabyah/jekyll,tomjohnson1492/jekyll,wentixiaogege/jekyll,isathish/jekyll,Teino1978-Corp/Teino1978-Corp-jekyll,ryanshaw/jekyll,ewinkler/jekyll,EddieDow/jekyll,garvitr/jekyll,rajrathore/jekyll,jaroot32/jekyll,kuangyeheng/jekyll,indraj/jekyll,nateberkopec/jekyll,mrb/jekyll,tamouse/jekyll,nickg33/jekyll,mnuessler/jekyll,honger05/jekyllcn,AnanthaRajuC/jekyll,nasht00/jekyll,jeffkole/jekyll,anthonyrosengren/grasshoppergathering2.github-io,xiongjungit/jekyll,ZainRizvi/jekyll,iRoxx/jekyll,stomar/jekyll,eninya/jekyll,ReachingOut/jekyll,cyberid41/jekyll,jekyll/jekyll,mrb/jekyll,Strangehill/jekyll,UniFreak/jekyllcn,tobscure/jekyll,alex-kovac/jekyll,julienbourdeau/jekyll,tugberkugurlu/jekyll,dangaute/eng,chiyodad/jekyll,lcustodio/lcustodio.github.io_base,stevecrozz/jekyll,superve/jekyll,AtekiRyu/jekyll,johnnycastilho/jekyll,ajhit406/jekyll,noikiy/jekyll,shliujing/jekyll,wentixiaogege/jekyll,thetaxman/jekyll,aarvay/jekyll,shliujing/jekyll,nemotan/jekyll,penchan1218/jekyllcn,alfredxing/jekyll,dansef/jekyll,fabulousu/jekyll,mnuessler/jekyll,winndows/jekyll,krishnakalyan3/jekyll,rongl/jekyll,tillgrallert/jekyll,yihangho/jekyll,leichunxin/jekyll,zsyed91/jekyll,oresmus/jekyll,MauroMrod/jekyll,rasa2011/rasa2011.github.io,xtymichael/jekyll,jaybe-jekyll/jekyll,xcatliu/jekyllcn,shuber2/jekyll,ducktyper/jekyll,princeofdarkness76/jekyll,stevecrozz/jekyll,julienbourdeau/jekyll,nasht00/jekyll,naughtyboy83/jekyll,goodtouch/jekyll,rlugojr/jekyll,eninya/jekyll,brint/jekyll,jeffkole/jekyll,akoeplinger/jekyll,xcatliu/jekyllcn,sasongkojati/jekyll,AnanthaRajuC/jekyll,rovrevik/jekyll,zhangkuaiji/jekyllcn,fulldecent/jekyll,rovrevik/jekyll,naughtyboy83/jekyll,jaybe-jekyll/jekyll,pmarsceill/jekyll,CheekyFE/jekyll,PepSalehi/jekyll,vidbina/jekyll,adilapapaya/jekyll,sasongkojati/jekyll,tomjohnson1492/jekyll,Strangehill/jekyll,schaazzz/jekyll,tomjohnson1492/jekyll,0x00evil/jekyll,thetaxman/jekyll,xiongjungit/jekyll,eloyesp/jekyll,ReachingOut/jekyll,alex-kovac/jekyll,kuangyeheng/jekyll,FlyingWHR/jekyll,rlugojr/jekyll,jmhardison/jekyll,isathish/jekyll,tamouse/jekyll,omeripek/jekyll,jmepg/jekyll,Tiger66639/jekyll,xtymichael/jekyll,18F/jekyll,darwin/jekyll,pmarsceill/jekyll,MjAbuz/jekyll,eloyesp/jekyll,ryanshaw/jekyll,tylermachen/jekyll,xantage/jekyll,fengsmith/jekyllcn,bsmr-ruby/jekyll,Teino1978-Corp/jekyll,bsmr-ruby/jekyll,AaronSikes/jekyll,0x00evil/jekyll,teju111/jekyll,UniFreak/jekyllcn,chinayin/jekyll,nathanhorrigan1/webdeveloper,nateberkopec/jekyll,gkunwar/gkunwar.github.io,Teino1978-Corp/Teino1978-Corp-jekyll,zsyed91/jekyll,mrb/jekyll,htmelvis/jekyll,Tyrion22/jekyll,isathish/jekyll,krahman/jekyll,larryfox/jekyll,ZDroid/jekyll,thejameskyle/jekyll,jeffkole/jekyll,ZDroid/jekyll,jaroot32/jekyll,xiaoshaozi52/jekyll,likong/jekyll,ryanshaw/jekyll,nathanhorrigan1/webdeveloper,imaustink/jekyll,goragod/jekyll,liukaijv/jekyll,fengsmith/jekyll,AtekiRyu/jekyll,gynter/jekyll,ls2uper/jekyll,ramoslin02/jekyll,royalwang/jekyll,greent2008/jekyll,Tiger66639/jekyll,wadaries/jekyll,0x00evil/jekyll,marrujoalex/jekyll,adilapapaya/jekyll,chrisfinazzo/jekyll,darwin/jekyll,johnnycastilho/jekyll,zsyed91/jekyll,supriyantomaftuh/jekyll,hiteshsuthar/jekyll,brint/jekyll,18F/jekyll,ls2uper/jekyll,likong/jekyll,lazycai/lazycai.github.com,likong/jekyll,QuinntyneBrown/jekyll,QuinntyneBrown/jekyll,xiebinhqy/jklly_china,rongl/jekyll,lazycai/lazycai.github.com,howkj1/jekyll,iRoxx/jekyll,superve/jekyll,AtekiRyu/jekyll,howkj1/jekyll,xiaoshaozi52/jekyll,alihalabyah/jekyll,xantage/jekyll,dansef/jekyll,tillgrallert/jekyll,chrisfinazzo/jekyll,nickg33/jekyll,SQS2/Test,Strangehill/jekyll,leichunxin/jekyll,lazycai/lazycai.github.com,christophermanning/jekyll,krahman/jekyll,AnanthaRajuC/jekyll,jmknoll/jekyll,Teino1978-Corp/jekyll,alfredxing/jekyll,dangaute/eng,FlyingWHR/jekyll,PepSalehi/jekyll,leichunxin/jekyll,hiteshsuthar/jekyll,m2candre/jekyll,stevecrozz/jekyll,derekgottlieb/jekyll,hubsaysnuaa/jekyll,tomjohnson1492/jekyll,derekgottlieb/jekyll,matuzo/jekyll,royalwang/jekyll,ajhit406/jekyll,oresmus/jekyll,desidude03/jekyll,shliujing/jekyll,princeofdarkness76/jekyll,abhilashhb/jekyll,jmepg/jekyll,hubsaysnuaa/jekyll,larryfox/jekyll,garvitr/jekyll,chinayin/jekyll,lcustodio/lcustodio.github.io_base,krishnakalyan3/jekyll,indraj/jekyll,EddieDow/jekyll,floydpraveen/jekyll,jekyll/jekyll,jmknoll/jekyll,goragod/jekyll,trungvothanh/jekyll,imaustink/jekyll,harrissoerja/jekyll,rebornix/jekyll,MjAbuz/jekyll,shuber2/jekyll,liukaijv/jekyll,kimeng/jekyll,gaosboy/jekyll,angeliaz/jekyll,schaazzz/jekyll,Teino1978-Corp/jekyll,backendeveloper/jekyll,mixxmac/jekyll,tobscure/jekyll,teju111/jekyll,supriyantomaftuh/jekyll,lazycai/lazycai.github.com,julienbourdeau/jekyll,daniel-beck/jekyll,wentixiaogege/jekyll,dezon/jekyll,drobati/jekyll,jaybe-jekyll/jekyll,dezon/jekyll,howkj1/jekyll,1gitGrey/jekyll,ducktyper/jekyll,schaazzz/jekyll,xcatliu/jekyllcn,chiyodad/jekyll,stomar/jekyll,LeuisKen/jekyllcn,ramoslin02/jekyll,kinlane/jekyll,daniel-beck/jekyll,alfredxing/jekyll,sasongkojati/jekyll,winndows/jekyll,QuinntyneBrown/jekyll,bsmr-ruby/jekyll,dansef/jekyll,rebornix/jekyll,vidbina/jekyll,rebornix/jekyll,egobrightan/jekyll,alfredxing/jekyll,richsoni/jekyll,alihalabyah/jekyll,nateberkopec/jekyll,rajrathore/jekyll,abhilashhb/jekyll,supriyantomaftuh/jekyll,egobrightan/jekyll,tugberkugurlu/jekyll,tillgrallert/jekyll,digideskio/jekyll,harrissoerja/jekyll,x-way/jekyll,sanxore/sanxore.github.com,princeofdarkness76/jekyll,tamouse/jekyll,mixxmac/jekyll,xiebinhqy/jklly_china,monxim/monxim.github.io,UniFreak/jekyllcn,AaronSikes/jekyll,zsyed91/jekyll,gaosboy/jekyll,matuzo/jekyll,mrb/jekyll,pmarsceill/jekyll,rlugojr/jekyll,jaybe-jekyll/jekyll,trungvothanh/jekyll,fengsmith/jekyll,htmelvis/jekyll,FlyingWHR/jekyll,omeripek/jekyll,martynbm/jekyll,lcustodio/lcustodio.github.io_base,nasht00/jekyll,yihangho/jekyll,derekgottlieb/jekyll,foocoder/jekyll,MauroMrod/jekyll,alex-kovac/jekyll,m2candre/jekyll,larryfox/jekyll,christophermanning/jekyll,noikiy/jekyll,tareq-s/jekyll,dangaute/eng,codeclimate-testing/jekyll,tylermachen/jekyll,tomjohnson1492/jekyll,backendeveloper/jekyll,matuzo/jekyll,foocoder/jekyll,jmptrader/jekyll,royalwang/jekyll,getandpost/jekyll.github.io,jaroot32/jekyll,thejameskyle/jekyll,Tyrion22/jekyll,hubsaysnuaa/jekyll,rlugojr/jekyll,Kinghack/kinghack.github.com,penchan1218/jekyllcn,foocoder/jekyll,nickg33/jekyll,digideskio/jekyll,kuangyeheng/jekyll,greent2008/jekyll,Kinghack/kinghack.github.com,yhironaka/yhironaka.github.io,CheekyFE/jekyll,richsoni/jekyll,pmarsceill/jekyll,anthonyrosengren/grasshoppergathering2.github-io,brint/jekyll,jmptrader/jekyll,bsmr-ruby/jekyll,darwin/jekyll,noikiy/jekyll,x-way/jekyll,LuoPX-15510486969/jekyll,penchan1218/jekyllcn,Strangehill/jekyll,cyberid41/jekyll,1gitGrey/jekyll,tomasdiez/jekyll,alex-kovac/jekyll,jmhardison/jekyll,goodtouch/jekyll,ryanshaw/jekyll,saitodisse/jekyll,sanxore/sanxore.github.com,tillgrallert/jekyll,rongl/jekyll,gynter/jekyll,jeffkole/jekyll,superve/jekyll,codeclimate-testing/jekyll,christophermanning/jekyll,doubleday/jekyll