Support for subtitle field (TIT3)

[daniele-athome]

This PR adds support for a subtitle field for MP3 (ID3v2 "TIT3") and ASF.

I couldn't find a reference for an MP4 tag.

Required by beetbox/beets#5696

[daniele-athome]

Modifying the library.py file worked (added the new field to Item._fields, opened beetbox/beets#5696 for that), but the new field is detected (and stored) only after an import. Do I have to re-import all of my library? Is there a quicker, more selective way maybe?

[jackwilsdon]

beet update might work.

[daniele-athome]

Thanks. Unfortunately it doesn't detect it, not even after touching the file. I could script it (i.e. a beets command plugin or even a shell script with some piped magic), but I hoped there was a faster way :-)