Add command core plugin.

This emits an on_command event when a user speaks to the bot in a
private message. Command arguments are parsed using shlex.
Catch BrokenImplementation exception.

If an interface method is missing completely, PluginManager.emit would
raise an unhanded exception.
Update auto_join config upon join/leave.

Also, auto-rejoin when kicked.
Basic bot control.

Tell the bot to join, leave or say something.