Allow leaving private messages

[dchiquito]

The /leave command is now permitted on private messages.

A settings.yml file is now created in the ~/.cabal/ directory. Right now it looks like this:

{cabal id}:
  joinedPrivateMessages:
    - {pm id}
    - {pm id}

Private messages are now only considered "joined" if they are present in the list in the settings file for the cabal. New PMs or /joining an old PM will add the PM id to the list, while /leave will remove it.

[cblgh]

@dchiquito woah, so fast! 💨

from what i can see: all looks good to me! i'll hold off merging until i can verify everything works as expected, which i'll do by way of implementing pm closing in cabal-cli using this branch's version of cabal-client; at the latest expect it next week :)

nicely done :>