This adds two new functions:
 * vorbis_comment_rm
 * vorbis_comment_rm_tag

These allow the removal of tags from a vorbis_comment structure.
vorbis_comment_rm allows to remove a tag selected by a provided string
in format "KEY=Value". vorbis_comment_rm_tag implements removal using
seperate strings for key and value. If no value is given the first
comment using the key is removed.