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 deleted the line because it didn't seem to do anything -- I'll doublecheck the code and make sure this is the case or restore the line.