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.
We could make these messages somewhat more succinct by formatting their values nicely: bitrate could be formatted as kbps, and length could use `human_seconds` so it looks more like a time.