Add separate storage styles for vorbis comments

Vorbis comments have a (very) small list of standardized tag names at
[0].

The differences between `vorbis` and `etc` are:
* trackc, discc, "album artist" and encoder have been removed
* musicbrainz_album{type,status} have been renamed to
  release{type,status}

The tag names for vorbis have been taken from [1] and [2].

[0] https://www.xiph.org/vorbis/doc/v-comment.html
[1] https://wiki.musicbrainz.org/PicardTagMapping
[2] https://docs.google.com/spreadsheet/ccc?key=0AoYQhIuWC-gcdHZwUDM3RGhneGMxSXpEX3pONXd6MWc#gid=0

This would fix #350 for vorbis comments by simply writing only one tag containing the track number.