Adds support for SoundCheck

This works, but it's probably a bit messy. You will probably want to refactor it somehow. It's working well for me, but it could definitely use some more testing.

* Adds 2 functions to convert between SoundCheck and ReplayGain
* Adds relevant StorageStyle to rg_track_* for SoundCheck
* Adds a new packing type for SoundCheck
* Modifies StorageStyle to accept a lang argument
* Modifies StorageStyle to allow specifying Packed out_type
* Modifies MediaField to write COMM frames with a lang attribute