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).
This is probably unnecessary now.  During my initial work, process was not included with ghcjs (or perhaps the wrong version was included), so it needed to be explicitly passed in.