Feature request: Union types

[SimenB]

I'd like to have a type called e.g. styling which searches css, sass, less and stylus, as well as a type for combined styling, js and json called e.g. frontend.

I could define those as their own types, but if I could compose other types together it would mean that if I add a new css dialect to styling, my frontend type would automatically be updated to use it.

[SimenB]

Hmm, seems to be a dupe of #83, which had some traffic less than a week ago. Promising!