added sloc

I packed the node cli app sloc [sloc](https://github.com/flosse/sloc) with npm2nix and `nodePackages.buildNodePackage`.  However there is an alternative way to do, e.g. like @matejc [tried it](https://gist.github.com/matejc/54a793979455c265baaa). Which way do you recommend for the future?
just use `coffee` not `${nodePackages.coffee-script}/bin/coffee` because you already specified it in `buildInputs` because you add `coffee-script` from `pkgs.nodePackages` and then you do not need it and use the other version inside `nodePackages` (thats what @7c6f434c  is talking about) ... `pkgs.nodePackages != nodePackages`