vorbis comment: keep PERFORMER away from ARTISTs
opus: granule positions are 48KHz
flac: skip over cover image properly
m4a: support multi-track files as well
vorbis: skip trying to read tags when there is none
Revert "try another license"

This reverts commit bc44b973cc0b141f29fc9417350f2ca14fe70441.

don't ignore valid audio files without tags
id3v2: make mp3 format check better
extension shouldn't matter, remove this logic and add Funknown
id3v2: add a note for possibly reading TOC in the future
id3v2: don't stop at false frame sync
move stuff around and clean up
change callback api to include raw "key" of the tag, add Tunknown type. this allows programs to read ALL text tags