github: Move IRC channel registration out of config parser.

This is better separation of concerns and allows the config parser to be
used outside of a harold context more readily.
plugin: Make it possible to declare dependencies optional.

Giving a default value to an argument of a plugin's `make_plugin` makes
that dependency optional. If the plugin resolver can't find an
appropriate plugin of that name, it will give `make_plugin` the default
value instead.
.