Add identification of audio CDs

[bitsgalore]

ISRC code: https://en.wikipedia.org/wiki/International_Standard_Recording_Code

Can be identified using: http://jonnyjd.github.io/musicbrainz-isrcsubmit/

But see also:

https://hydrogenaud.io/index.php/topic,98359.0.html

Which suggests that esp. older CDs often don't have ISRCs.