Support for new twitch capabilities

[richardbenson]

Fix for #76

Twitch now support capabilities, and have three available (https://dev.twitch.tv/docs/irc#twitch-capabilities) but they don't support multi-prefix (not does the channel plugin work)

This PR allows capability setting when daemon is set to twitch and doesn't require multi-prefix

[richardbenson]

So just bumped up against this again, the above fix still appears to work just fails tests because the tests don't take into account twitch supporting capabilities. Any chance of this being looked at?

[VictorioBerra]

@richardbenson is this something you still need/wanted?

[richardbenson]

@VictorioBerra I still use Tennu and still with the same change made manually every time I update, I don't know how useful it is to others but without this patch you can't do anything interesting with Twitch's IRC.