Flake support

Closes #200 (closed).

This should be backwards-compatible. I didn't touch the tests directory because I wasn't entirely sure how to convert it. I was thinking we could use the checks flake output (see here for an example) to provide the intern, extern, clamav and multiple attributes, and the user could test against a different version of nixpkgs using something like

nix flake check --override-flake nixpkgs nixpkgs/release-20.03

But that's not backwards-compatible, of course.

EDIT: actually, I guess the better solution would be to use the hydraJobs flake output.