Add extra replaygain fields from loudgain

[Moonbase59]

As discussed in forum, here are the new MediaFile fields to make magic happen in beets.

[jtpavlock]

I think I'm a bit confused, is loudgain supported by beets, or is this PR attempting to preemptively add fields for an unsupported backend component?

[sampsyo]

Indeed; as I mentioned recently in #3368, I've sort of lost the context here.

[jtpavlock]

In that case I'm closing this for now. I think it would also make sense to couple this change with the actual implementation of loudgain

[jtpavlock]

Looks like beetbox/mediafile#19 is relevant, which provides the implementation of these fields, but still doesn't make sense unless we actually switch to or add loudgain as a replay gain backend.