[Feature Request] Add support for the 'DiscSubTitle' metadata tag

[OttoKerner]

In order for plexamp to be able to show this, it must first be read in by Plex server.
So this is a feature request for Plex server, not for plexamp. And it already exists.

Non-numerical labels for discs

I have a pretty big library, and for some items, there are multi-part albums, such as Whitney Houston’s “The Thrown Down & the Cool Down” or Foo Fighter’s “In Your Honor” or Jars Of Clay’s “Furthermore”, as well as audiobook collections, such as Peter F. Hamilton’s Commonwealth saga, or George RR Martin’s A Song of Ice and Fire sage (or The Expanse, or Mistborn, or various others). For these, I’d like to keep the collections together. The ID3 tags don’t have standard Collection support, but the…