Add Meslo LG & Meslo LG DZ fonts

This is a neat little font set which is missing from the package repository. It is absolutely great for coding.

As per the [discussion](https://github.com/balajisivaraman/nixpkgs/commit/425e6965df82f381686f7c445b96b2a71fec207a#commitcomment-8315049) with @Fuuzetsu, I've updated the package to use `stdenv.mkDerivation` instead of the old `builderDefsPackage`. It seems a lot cleaner now and builds without any errors as well.
"Don’t repeat the name of package. Don’t include a period at the end." http://nixos.org/nixpkgs