Manage a global package environment for easy GHCi access

See https://www.reddit.com/r/haskell/comments/507h0p/cabal_newbuild_is_a_package_manager/d72bzce?context=3

I think, essentially, the request is for `cabal new-install` to put the requested package into some global packages set, which we keep up-to-date upon invocations to `new-install`. Maybe we should even write the package environment in the place GHC looks by default, so a bare `ghci` invocation works.

@hvr thought `extra-packages` might be what you want (if it were implemented) but I'm not sure; I think it would cause any such mentioned packages to be considered local, and thus built inplace. But maybe that's a misinterpretation.

CC @cocreature