Add new iTunes MP4 tags

[Matthew Meissner]

As included in the macOS Sierra beta version, iTunes 12.5 will support five new tags to better classify classical music. It would be great if Subler could add support for these tags.

• ©wrk: Work (string)
• ©mvn: Movement Name (string)
• ©mvi: Movement Number (number)
• ©mvc: Movement Count (number)
• shwm: Show Work & Movement (0/1)