faster riot.mount.selctAllTags
faster riot.mount.selctAllTags

riot.mount.selctAllTags is slow, this alternative function is 55% faster.
In rare cases, cache in allTags avoids unnecessary call.

See [the perf test](http://jsperf.com/riot-alternative-selctalltags)

davidmarkclements/riot,mike-ward/riot,rthbound/riot,xtity/riot,sylvainpolletvillard/riot,thepian/riot,joshcartme/riot,crisward/riot,txchen/riotjs,dp-lewis/riot,beni55/riot,laomu1988/riot,dp-lewis/riot,scalabl3/riot,stonexer/riot,crisward/riot,chetanism/riot,xieyu33333/riot,scalabl3/riot,baysao/riot,stonexer/riot,xieyu33333/riot,qrb/riotjs,rthbound/riot,antonheryanto/riot,noodle-learns-programming/riot,laomu1988/riot,chetanism/riot,antonheryanto/riot,seedtigo/riot,dschnare/riot,sylvainpolletvillard/riot,a-moses/riot,txchen/riotjs,ListnPlay/riotjs,dschnare/riot,tao-zeng/riot,rasata/riot,AndrewSwerlick/riot,duongphuhiep/riot,xtity/riot,davidmarkclements/riot,madlordory/riot,mike-ward/riot,seedtigo/riot,zawsx/js-lib-webcomponents-react-riot,marcioj/riot,zawsx/js-lib-webcomponents-react-riot,daemonchen/riotjs,muut/riotjs,baysao/riot,ListnPlay/riotjs,joshcartme/riot,thepian/riot,beni55/riot,tao-zeng/riot,marcioj/riot,daemonchen/riotjs,qrb/riotjs,rasata/riot,duongphuhiep/riot,a-moses/riot,AndrewSwerlick/riot,noodle-learns-programming/riot,muut/riotjs,madlordory/riot