Add message_format option

Noticed that the default "html" message_format doesn't allow emoticons to display properly in Mac client 2.3, though the emoticons will display in Linux client. According to https://www.hipchat.com/docs/api/method/rooms/message, html format doesn't allow for emoticons (so there's some inconsistent behavior here).

That said, seemed like this should be an option regardless -- let me know if anything should be changed!