Display multiple labels/catalog numbers where applicable

[emiham]

Currently, beets only shows one catalog number, which is probably correct for a majority of releases. Some releases, however, have more than one label and more thus more than one catalog number. In a lot of cases you might only have one of these visible where you got the album from, so it would be useful to be able to see all of them during an import.

I know it's a goal to keep the importer output short, so it might just be that this would make it too long. In that case maybe this would be more suitable for a plugin?