Read ISRCs from CUE files

[JonnyJD]

Requested in JonnyJD/musicbrainz-isrcsubmit#42

That obviously would make use of DiscId.put(), but we might even implement gathering the data from the CUE files in the discid module.

That would be the first function that is not just forwarded to libdiscid.

[JonnyJD]

We have to check if we get the same ID, even for data CDs. Possibly we need to detect data CDs and handle them differently.

[JonnyJD]

Another option might even be to implement that part in libdiscid, when special handling is necessary.

[JonnyJD]

Not much special handling is necessary. We just have to make sure that we only input last AUDIO track and don't append an offset for the (appended) data track.

We could possibly still include that in libdiscid, although parsing files is easier in python, I guess.

[JonnyJD]

cue sheets alone are not enough. They are missing overall length. kepstin/image-id#1 might be a better place to implement this.