Added CartoCSS to languages.yml

I only know about a [grammar for Atom](https://github.com/yohanboniface/carto-atom), that [looks pretty good](https://lightshow.githubapp.com/?utf8=%E2%9C%93&scope=from-url&grammar_url=https%3A%2F%2Fraw.githubusercontent.com%2Fyohanboniface%2Fcarto-atom%2Fmaster%2Fgrammars%2Fcarto.json&grammar_text=&code_source=from-url&code_url=https%3A%2F%2Fraw.githubusercontent.com%2Fgravitystorm%2Fopenstreetmap-carto%2Fmaster%2Flandcover.mss&code=).