Duplicates plugin

This plugin adds a new command, `duplicates` or `dup`, which finds
and lists duplicate tracks or albums in your collection.

By default, the `beet duplicates` command lists the names of tracks
in your library that are duplicates. It assumes that Musicbrainz track
and album ids are *unique to each track or album*. That is, it lists
every track or album with an ID that has been seen before in the
library.