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.
see what I said on IRC, unless the `$watch` on the noembed checkbox fires on load, we need this. If it reliably fires (across browsers/devices etc), then we'd be fine without it of course. I'm just not sure.