Implement extended comment syntax {{!-- --}}
Implement extended comment syntax {{!-- --}}

This syntax is available in handlebars.js, could be hidden behind a feature
flag if desired.

cgvarela/lightncandy,vein1992/lightncandy,Corollarium/lightncandy,jmgq/lightncandy,Southparkfan/lightncandy,zordius/lightncandy,cgvarela/lightncandy,zordius/lightncandy,Southparkfan/lightncandy,fadimko/lightncandy,Corollarium/lightncandy,fadimko/lightncandy,jmgq/lightncandy,vein1992/lightncandy