Add a ~/.owl/.irc.channels file, persist channels

All irc channel subscriptions persist across reconnects.

irc-{part,join} now have a -t flag, which makes the channels not persist
across BarnOwl sessions.

irc-loadchannels loads persistent channels.  It does not unload existing
channels.

TODO: make it so that :irc-join commands in ~/.owl/startup don't result
in lots of error messages in :show errors.  This currently occurs
because BarnOwl errors whenever you try to subscribe to an already
existing subscription (in zephyr), and I reused the zephyr machinery for
IRC.