Submit ISRCs from digital music files instead of CDs

[SheamusPatt]

I use isrcsubmit a lot, but more and more I'm getting my music digitally. Those files usually have embedded ISRCs. Instead of a CD, it would be great if I could pass along a list of music files with embedded ISRCs, which isrcsubmit could then submit to MusicBrainz.
This is similar to #42 and has the same issue - how to identify the release? One way is to rely on MusicBrainz tags, ie. MUSICBRAINZ_ALBUMID. If you run MusicBrainz Picard prior to isrcsubmit, you can deal with locating the proper release at that stage. In fact individual recordings could be targeted using MUSICBRAINZ_TRACKID allowing for an entire group of music files to have their codes submitted. As a fallback, isrcsubmit already has the --release-id parameter requiring only matching up track names to identify the right recording.