Support for subtitle field (TIT3) (#82)

This PR adds support for a subtitle field for MP3 (ID3v2 "TIT3") and
ASF. Reference for an MP4 tag missing.

Required by beetbox/beets#5696

Author: semohr

mediafile.py

@@ -1807,6 +1807,11 @@ def as_dict(self):
         StorageStyle('GROUPING'),
         ASFStorageStyle('WM/ContentGroupDescription'),
     )
+    subtitle = MediaField(
+        MP3StorageStyle('TIT3'),
+        StorageStyle('SUBTITLE'),
+        ASFStorageStyle('Subtitle'),
+    )
     track = MediaField(
         MP3SlashPackStorageStyle('TRCK', pack_pos=0),
         MP4TupleStorageStyle('trkn', index=0),

test/test_mediafile.py

@@ -1111,7 +1111,7 @@ def test_known_fields(self):
             ('encoder', 'images', 'genres', 'albumtype', 'artists',
              'albumartists', 'url', 'mb_artistids', 'mb_albumartistids',
              'albumtypes', 'catalognums', 'languages', 'artists_credit',
-             'artists_sort', 'albumartists_credit', 'albumartists_sort')
+             'artists_sort', 'albumartists_credit', 'albumartists_sort', 'subtitle')
         )
         self.assertCountEqual(MediaFile.fields(), fields)