Enhancement: support for tagging for m4a / aac / mp4 formats

[quiescer]

I converted my whole music collection to .m4a a while ago.

[hellricer]

Hey, I didn't even know that mid3v2 supports these!

There's gonna be some refactoring needed, but I'll try to do my best.

[hellricer]

So, turns out that while mid3v2/mutagen is capable of saving ID3 tags into other formats as well, it appears that most of programs happily ignore those in favor of native tags (flac/ogg use Vorbis Tag, mp4/m4a/aac use completely different one too).

That means that the way the data are read/saved needs to be abstracted, so other programs like metaflac & ogginfo can be used... Not sure there's any decent console tool for microsoft formats, though.