add a flag/configuration option to skip importing/ignore an album if a specified condition is met

[amogus07]

Currently, when re-importing items already in the library (e.g. to fetch missing/updated metadata), one can use a query to filter the local database. It would be very helpful to filter metadata from remote sources the same way. An example: beets import -L --ignore='bandcamp_album_id:=""' data_source:=MusicBrainz would skip importing albums whose metadata from MusicBrainz has an empty bandcamp_album_id field.