Add MusicBrainz IDs as options for Format Specifier

It would be nice if we could use the MusicBrainz IDs that are generated for a given release (artist, album, track, etc) as Format Specifiers for file and folder naming. I store all my flac images in a hierarchy of MB UUIDs because that will never change. (In theory, textual metadata could be edited post-submission) This naming convention makes it easier for collection databases to manage images since the filenames will never change, even if the metadata does.