I think we should keep the label names to what it currently is (because it is a sane default).

The interactivity happens mostly in `sup-config`, so we should tell the user that some folders are mandatory and how they are used, so he can either create them through `sup-config` or abort sup-config and create them manually (use case: importing mails from an existing hierarchy). He can also symlink to his existing dir if he doesn't like the name.

But I guess we should leave that interactivity in `sup-config` and keep the lib non-interactive -- it is not expected to be called directly anyway.