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).
Yeah, HEAD is a pretty good choice. Alternatively, the output from 

    $ git describe --tags 
    cabal-install-v1.20.0.3-507-g699d4df

suggests `Cabal-1.20.0.3-507-g699d4df`. I dunno ... HEAD is probably better, but don't feel strongly about it.