Add FIRST_RELEASE_DATE tag

[zoomorph]

If the release has a master release, this tag will contain the earliest release date from that master release.

[Jan Musmann]

I'm really looking forward for this tag. Will it also be available in chunks (year, month, day) like the normal release date? Would be a real treat if so! Hope you find some time to implement it soon.

[zoomorph]

In the upcoming version 2.0, MASTER_RELEASE_DATE contains the date of the master release, which should be the first release date. Day and month would also be available if available on Discogs. Prototype can be found on the hydrogenaudio thread.