add Replay Gain volume tags to Ogg Vorbis files

 vorbisgain calculates a percieved volume of an Ogg Vorbis file using
 the Replay Gain algorithm. It then stores a per-track and per-album
 volume adjustment in the file's tags, to let songs play back with a
 uniform volume. The process is non-destructive and does not change the
 audio data at all.
 .
 Many audio players in Debian support reading these tags, including XMMS,
 Quod Libet, amaroK, Muine, and Rhythmbox.