Basic summary information for duplicates

Added a one-line summary of each album (in lib, and import target) so
you can easily tell if (for example) you are about to overwrite your
FLAC copy with a low-bitrate mp3 from somewhere else.
Instead of adding this as a field on the Task, can we instead just pass it functionally along to `resolve_duplicate` as a parameter? This will make maintaining & testing much easier—the `task.find_duplicates()` call in `beets.importer.resolve_duplicates()` could just pass its return value into `resolve_duplicate` and avoid the need for mutation.