Put default IRC settings in shared config.

I plan to implement the Twitter -> ArchiveBot interface as a separate
bot because I don't like the idea of polluting up the existing bot with
a bunch of Twitter-specific stuff.  To do this, it'd be nice to have all
the default IRC connection settings in one place.
Introduce TwitterConcierge.

The TwitterConcierge takes requests from the TwitterListener, relays
them to ArchiveBot, and then takes care of translating ArchiveBot status
updates to replies to the original Twitter-based request.