Add some various fixes

These are needed for new users who want to convert a partially existing maildir root into a sup's maildir root.

- add the possibility to configure a maildir-root in `sup-config`.
- minor addition in sup-config : loop on user input until the given path actually exists (helps prevent typos earlier)
- skip folders that aren't real maildirs, so that it doesn't break at polling time
- create special labels' folders if they don't exist. I hope it's an OK thing to do ?
- Remove an annoying warning about `Enumerator.new` being deprecated