add support for breakout IRC channels

[dbaron]

In 898206e I removed the ability to /invite the bot to a new channel, because it wouldn't know which channel's configuration was relevant.

However, it seems like it could make sense to readd this if, instead, it were a breakout command that could be given to the bot in a channel in which it already had a configuration.

[dbaron]

(and in this case it should probably time out and leave the breakout channel at some point, rather than just waiting until its next reboot)