windowManager.xmonad: Make it configurable and extensible.

The previous windowManager.xmonad option only starts xmonad and
doesn't make ghc available. This assumes that the user has GHC with
access to the xmonad package in his PATH when using xmonad.

Xmonad in Nix is now patched to accept the XMONAD_{GHC,XMESSAGE}
environment variables which define the path to either ghc or xmessage.
These are set automatically when using xmonad through
windowManager.xmonad.

My (or specific: @aristidb and my) changes make it possible to use
Xmonad without adding GHC to any profile. This is useful if you want
to add a different GHC to your profile.

This commit introduces some options:

- xmonad.haskellPackages: Controls which Haskell package set & GHC set
  is used to (re)build Xmonad

- xmonad.extraPackages: Function returning a list of additional
  packages to make available to GHC when rebuilding Xmonad

- xmonad.enableContribExtras: Boolean option to build xmonadContrib
  and xmonadExtras.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>