cabal2nix: improve flexibility of the generated shell.nix file
cabal2nix: improve flexibility of the generated shell.nix file

The output produced by "cabal2nix --shell" is now more versatile:

 - Running "nix-shell" will enter an interactive environment that's suitable
   for compiling the generated Haskell package.

 - Running "nix-build shell.nix" will compile the package with Nix. (Note that
   you may have to clean the "dist/" directory for these builds to succeed.)

 - Either command can be passed an argument "--argstr compiler ghcXYZ" to
   determine the Haskell package set to use for the build. For example,
   "--argstr compiler ghc784" instantiates the expression to use GHC 7.8.4. The
   default choice is "ghc7101".

bennofs/cabal2nix,spencerjanssen/cabal2nix,psibi/cabal2nix,Fuuzetsu/cabal2nix,gridaphobe/cabal2nix,dmalikov/cabal2nix,jb55/cabal2nix