Add support for GHCJS

This provides a (AFAICT) fully working GHCJS package set.

@peti Can I get your feedback on how to best merge this in? I'm sure there are some things to consider, like if/how we should specify that a given package supports GHCJS (or more generically, any particular compiler).
Why do you introduce those additional arguments to the function instead of simply overriding the `cabal` attribute to point to the ghcjs builder in the configuration function for the ghcjs package set? The same goes for the `wrapper`?