allow override page url and save_as values directly from the metadata
allow override page url and save_as values directly from the metadata

this is similar to the template override implemented in #404

talha131/pelican,liyonghelpme/myBlog,abrahamvarricatt/pelican,ehashman/pelican,eevee/pelican,abrahamvarricatt/pelican,karlcow/pelican,gymglish/pelican,koobs/pelican,HyperGroups/pelican,levanhien8/pelican,TC01/pelican,ls2uper/pelican,btnpushnmunky/pelican,ingwinlu/pelican,jo-tham/pelican,justinmayer/pelican,lazycoder-ru/pelican,karlcow/pelican,51itclub/pelican,alexras/pelican,lucasplus/pelican,Scheirle/pelican,11craft/pelican,liyonghelpme/myBlog,lucasplus/pelican,arty-name/pelican,number5/pelican,0xMF/pelican,rbarraud/pelican,btnpushnmunky/pelican,kennethlyn/pelican,ehashman/pelican,liyonghelpme/myBlog,zackw/pelican,sunzhongwei/pelican,karlcow/pelican,HyperGroups/pelican,deved69/pelican-1,ls2uper/pelican,JeremyMorgan/pelican,TC01/pelican,btnpushnmunky/pelican,liyonghelpme/myBlog,alexras/pelican,lucasplus/pelican,UdeskDeveloper/pelican,alexras/pelican,levanhien8/pelican,crmackay/pelican,11craft/pelican,florianjacob/pelican,koobs/pelican,rbarraud/pelican,kennethlyn/pelican,zackw/pelican,iKevinY/pelican,simonjj/pelican,iurisilvio/pelican,catdog2/pelican,getpelican/pelican,Summonee/pelican,fbs/pelican,goerz/pelican,avaris/pelican,garbas/pelican,UdeskDeveloper/pelican,Scheirle/pelican,douglaskastle/pelican,florianjacob/pelican,simonjj/pelican,farseerfc/pelican,JeremyMorgan/pelican,ls2uper/pelican,jimperio/pelican,sunzhongwei/pelican,UdeskDeveloper/pelican,JeremyMorgan/pelican,crmackay/pelican,goerz/pelican,levanhien8/pelican,gymglish/pelican,gymglish/pelican,joetboole/pelican,janaurka/git-debug-presentiation,treyhunner/pelican,rbarraud/pelican,treyhunner/pelican,iKevinY/pelican,janaurka/git-debug-presentiation,ingwinlu/pelican,TC01/pelican,joetboole/pelican,Polyconseil/pelican,abrahamvarricatt/pelican,jvehent/pelican,jvehent/pelican,Rogdham/pelican,deanishe/pelican,11craft/pelican,number5/pelican,sunzhongwei/pelican,talha131/pelican,douglaskastle/pelican,Polyconseil/pelican,sunzhongwei/pelican,HyperGroups/pelican,jimperio/pelican,ehashman/pelican,deved69/pelican-1,51itclub/pelican,simonjj/pelican,goerz/pelican,Summonee/pelican,Rogdham/pelican,janaurka/git-debug-presentiation,Rogdham/pelican,crmackay/pelican,florianjacob/pelican,kernc/pelican,deved69/pelican-1,GiovanniMoretti/pelican,douglaskastle/pelican,ionelmc/pelican,kernc/pelican,treyhunner/pelican,Scheirle/pelican,joetboole/pelican,51itclub/pelican,koobs/pelican,liyonghelpme/myBlog,iurisilvio/pelican,farseerfc/pelican,Summonee/pelican,jimperio/pelican,avaris/pelican,deanishe/pelican,garbas/pelican,eevee/pelican,jvehent/pelican,iurisilvio/pelican,catdog2/pelican,lazycoder-ru/pelican,kernc/pelican,garbas/pelican,kennethlyn/pelican,eevee/pelican,getpelican/pelican,zackw/pelican,GiovanniMoretti/pelican,number5/pelican,jo-tham/pelican,lazycoder-ru/pelican,GiovanniMoretti/pelican,deanishe/pelican,catdog2/pelican