Draft: Add support for nar files

I encounter nar files when working with GNU Guix, but the format comes from the Nix project.

This is the archive format used for Guix binary substitutes, so it's very useful to be able to use diffoscope to compare nars (it's equivalent to comparing .deb's). Currently though, you have to unpack the nar first, then run diffoscope on the directories. For simplicity, I've been looking at whether diffoscope could take care of that.