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).
@ryantrinkle Ok, done in https://github.com/NixOS/nixpkgs/commit/780ec20283380bf0524978d03bd67d4b4aec1250.