Feature Request: Replaygain support.

[Replaygain is a system that audio players use to ensure consistent percieved loudness amongst tracks and albums](https://wiki.hydrogenaud.io/index.php/ReplayGain)

This would be incredibly useful for managing the percieved loudness of voice acting: A replaygain scanner can easily be found in an audio player like Foobar2000, can handle hundreds of files very quickly - far more quickly and smoothly then manually having to normalize the audio of each file. And since it only adjusts the metadata of the files, the contents of the audio itself doesn't need to be adjusted, which ensures it's a reversible and easily configurable process.

It would also be useful for mods that use their own music, to ensure that tracks are consistent with eachother and FS2's baseline.

On the FSO side, there would have to be table settings for the following options:
Source: Whether none, track, or album gain is used
gain mode: Whether gain, prevent clipping, or apply gain and prevent clipping.
Pregain for files without replaygain: To control the relative volume of, say, FS2's files.
Gain for files with replaygain: The control the relative volume of the added files.

On the user end, the user would need a replaygain scanner to get it to work.