library: add subtitle (id3 TIT3) field

Author: daniele-athome

beets/library.py

@@ -546,6 +546,7 @@ class Item(LibModel):
         "work_disambig": types.STRING,
         "arranger": types.STRING,
         "grouping": types.STRING,
+        "subtitle": types.STRING,
         "year": types.PaddedInt(4),
         "month": types.PaddedInt(2),
         "day": types.PaddedInt(2),

docs/reference/pathformat.rst

@@ -218,6 +218,7 @@ Ordinary metadata:
 * genre
 * composer
 * grouping
+* subtitle
 * year, month, day: The release date of the specific release.
 * original_year, original_month, original_day: The release date of the original
   version of the album.