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).
Note that I added lens-4.6.0.1 in https://github.com/cstrahan/nixpkgs/commit/3b25e233ab7e13e1973b18e3d5ae4125166cfe3c because lens-4.4 didn't build because of an outdated text dependency.

I didn't bother to remove the lens-4.4 package because I don't know the policy regarding multiple package versions in nixpkgs and when to remove older versions. Where can I find some documentation on that?