Added CartoCSS to languages.yml

Oh cool, we could use that. Can you take a look at the section on [adding a new grammar](https://github.com/github/linguist/blob/master/CONTRIBUTING.md#syntax-highlighting-looks-wrong) and open a new pull request? We'll also need https://github.com/yohanboniface/carto-atom to add a license to their repo before we can add it.