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 derivation is clearly labelled as `cabal-install-1.22.0` according to the file name, yet that version number doesn't show up anywhere in the file and, in fact, is imported from another expression. This is going to lead to some surprises if the version number of `Cabal` ever changes. Maybe there should be some assertion in here to guarantee that we really have version 1.22.0?