supervisor::ctlplugin

adds `[ctlplugin:foo]` stanza defined types.

This is useful when setting up supervisord plugins that come with plugins for supervisorctl, like [mr.laforge](https://github.com/fschulze/mr.laforge)

This should probably get merged in along with #26 because adding these resources currently causes a full restart of supervisord, when it's actually not even really necessary to tell supervisord to reread its configs when these get added or removed.

Also, sadly, in my experience, these can't be placed in the conf.d directory because supervisorctl doesn't follow includes for some reason.