Add supported languages to package.json

See: https://github.com/adobe/brackets/wiki/Extension-package-format#details.
When you specify languages in that field, it will show a hint in the extension window like this:
![extension_rating_i18n](https://cloud.githubusercontent.com/assets/2564094/3931150/41af299c-2453-11e4-95b3-d87e8dfb578f.png)