Add BetterTTV support to beta chat

BetterTTV takes over the chat and uses their own event handler for new
messages. If we want to support BetterTTV users we have to test for the
presence of BTTV and override that function. Fixes #76 for the beta
chat, and we'll just say that BTTV support for the old chat is no longer
a thing.

As to whether or not this is necessary, we can leave that up to discussion.
There are 157,600+ users of BTTV and this doesn't add much code..