#78 Write ReplayGain frames to ID3v2 tag

[Jim McAtee]

An option to write computed ReplayGain values to TXXX=REPLAYGAIN_TRACK_GAIN and TXXX=REPLAYGAIN_TRACK_PEAK (with --replaygain-accurate) frames to ID3v2 is long overdue. The ability to compute RG is helpful, but there are still few (if any) applications that can use this information read from the LAME tag.

Keep the existing behavior, but add a command-line option to also write the values to the appropriate ID3v2 frames.