Add ".cson" to languages

> Same as JSON but for CoffeeScript objects.

See https://github.com/bevry/cson.

Would be really great to have this in `docco`, e.g. to document (`grunt`) build process configs like so
![screen shot 2014-03-26 at 10 15 41](https://cloud.githubusercontent.com/assets/425211/2534748/60848510-b581-11e3-93a4-376c627b400f.png)