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).
I completely agree.  This isn't a sustainable solution - I just wanted to hack out something to get it to a working state; I removed the comment because it was (subsequently) false.

Using extension to override the autogenerated package seems like the way to go; this package makes heavy use of cabal flags, which heavily modify its dependencies, so I think it would be very difficult for cabal2nix to deal with everything automatically.