Some more awesome players use a set of replaygain_* tags to adjust the
playback volume on the fly - without the need to modify the audio data
on disk. It's completely lossless.

However, to create these tags, you have to use a different specialized
tool for each different format. metaflac for flac files, vorbisgain for
ogg, etc.

This is just a wrapper script, in python, to call the various tools for
each type, so you don't have to think about it.

In the most simple case:
replaygain
will just take all taggable audio files in the current directory,
calculate batch gain values and individual gain values, and save the
tags into each file.

More complex operations could include things like:

replaygain -a
* For each distinct album tag found on all input files, calculate a
* separate batch album value for files with that album tag.

replaygain -r -d
* Find all files recursively under the given directory, calculate the
* batch values grouped by directory.

replaygain -r -a
* like the previous, except keep track of ALL files found recursively,
* their album tags, and the running batch average. Save it all at the
* very end.

replaygain *.flac
* Only operate on the files given

replaygain -t
* Separate batch calculation by filetype - you might have mp3 files from
* one source, and flac files from another, but all from the same album.
* It wouldn't necessarily make sense to calculate them as a single
* album.

When replaygain has a set of files in what it will consider an "album"
and all files have homogeneous replaygain_album_* tags, then replaygain
will skip them unless asked not to. It will also assume any existing
replaygain_track_* tags are correct unless asked not to. Relevant flags
to be determined.