Add support for highlighted messages

[izzy]

Currently it seems highlighted messages just get passed through to privmsg without any field for being highlighted. In the source you can see there is a msg-id field in tags filled with highlighted-message.

I couldn't find anything in the docs on Twitch's site about this but imo this should be treated similar to /me messages and get it's own bool field that defaults to false..?

Similarly it would probably be useful to include the tag for first-msg and returning-chatter, although I'm not sure how stable the latter is, it seems that is still experimental.