command listeners support

[pzduniak]

Depends on #127

Now that we're officially making ! commands "standard" it feels like we should support it out of the box in this library per the discussion in #165. Unfortunately this isn't really trivial because of chat listeners spawning a process each rendering the most basic approach (ie. spawn a new "chatListen" for every command handler) unusable due to performance penalties. This should be most likely done in a single push, together with #127.