Twemoji Support, Take 2

Similar to the PR from before ( #505 ), but now Twemoji parsing is optional and it defaults to disabled. I still plan  on adding UA detection for better default handling, but wanted to get _something_ out while I figure out the best way to do so.
if you go the parameter route outlined above, you can drop this and the changes below.

Why did you delete the `$rootScope.auto_display_embedded_content` line though?