opusenc: Add gain setting options

[9d8]

Added the options --output-gain, --track-gain, and --album-gain to opusenc to directly set the gain for an opus file.

I'm aware that opusenc can currently set the gain data of an opus if the input flac contains ReplayGain tags. Though this change makes it much easier to set the gain for files that don't have existing ReplayGain metadata.