Preserve files modification time

[drovnotya]

It's too late for my own use because I have run rsgain on my files already, but if this was an option I would have used it.

For instance metaflac has a --preserve-modtime, and also foobar2000 and Mp3tag (Windows software) has the ability to do the same when altering a files tags.

[complexlogic]

I'll look into this.

[complexlogic]

Implemented via 09f84d8