1. Switch to TITLE[n] style tags

1. Switch to TITLE[n] style tags
2. Support multi-artist discs

(tags_file): Split all the duplicated candidate-tags file writing
stuff from tags into this new function.  It uses TITLE[n] style tags,
and ARTIST[n] tags for multi-artist discs.

(tags): Use tags_file.  If any of the matching MusicBrainz album
objects' has_various_artists function returns true, make a
candidate-tags-0v file with blank ARTIST[n] entries, as well as
putting them in the candidate-tags-$i file.