feature request: Allow multiple --root arguments

I want to compare two closures and see what is shared and what is not. As far as I can tell, I can't do that with `nix-du` without first building a derivation that depends on both (or using nix-du on my entire system and filtering out the parts I'm interested in)? Using multiple `-r` arguments gives the error: ``` error: the argument '--root <PATH>' cannot be used multiple times ```