Support for GitHub/GitLab PR URLs

[Gabriella439]

One common use case for cabal2nix is using it to fetch a certain pull request for use as a dependency. However, currently one has to manually translate the PR URL to a revision before one can use cabal2nix. It would be nice if one could instead run something like:

$ cabal2nix https://github.com/andrewthad/country/pull/29

… and (at the time of this writing) it would behave the same way as if one had ran:

$ cabal2nix https://github.com/andrewthad/country --revision 4477749919da35e392faf08538f07c4c175b8284

I'd be happy to contribute this change myself if it would be approved. I've already looked into this and seemed like the most idiomatic way to do this would be to first contribute a nix-prefetch-pr utility to Nixpkgs and then make cabal2nix a thin wrapper around that. However, I wouldn't want to open the PR for nix-prefetch-pr if the matching change to cabal2nix would not get approved.