missing: Configuration option to exclude music

There are some albums where I specifically don't keep certain tracks.
Doing a search for missing tracks, these will always come up and clutter the list.

So, I propose a blacklist: If a track or album is in the blacklist, the `beet missing` list would simply not show them, or show a `+42 tracks blacklisted` indicator at the end of the list.

Start with the `beet missing` command and determine how a track or album blacklist should be configured and applied. Done means blacklisted tracks or albums no longer clutter the missing list, with the proposed aggregate indicator considered if retained.

Written by the indexing model from the issue text.