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`

`coffee` within `buildInputs` evaluates to coffee-script `v1.7.1` but I want to use `v1.8.0`. Therefore I use `nodePackages.coffee-script`. In fact for this case it doesn't make a difference but generally I'd like to use dependency versions that are recommended by the author.