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.
I dislike the coupling here. It would be nicer to pass this to the filter as a parameter, so it doesn't depend on `$rootScope` (filters should be reusable). You'd have to add parameter support to `DOMfilter` for that, but since we don't need anything fancy a simple pass-along parameter would be totally enough. I did that before for another filter that I played with but discarded the whole branch, sorry.