replaygain: Add a loudgain backend

### Use case

As the current developer of [loudgain](https://github.com/Moonbase59/loudgain), a ReplayGain 2.0 tagging tool for many file formats, I’d much like to see loudgain being supported by beets as an official ReplayGain backend.

### Solution

I assume integration shouldn’t be too complicated, since loudgain uses an enhanced version of the well-known _mp3gain_ commandline syntax.

Let me know how I could help getting this going.
Any testing results from beets users and feedback on loudgain’s [issue tracker](https://github.com/Moonbase59/loudgain/issues) welcome!

### More info

[loudgain on GitHub](https://github.com/Moonbase59/loudgain)
[loudgain in the Hydrogenaudio wiki](http://wiki.hydrogenaud.io/index.php?title=Loudgain)