cabal: add support for "editedCabalFile" attribute to address https://github.com/NixOS/cabal2nix/issues/84

When 'editedCabalFile' is set to an SHA256 hash, the build driver replaces the
original Cabal instructions with the new version that is downloaded from
"http://hackage.haskell.org/package/${fname}/${pname}.cabal".
haskell-twitter-conduit: use new editedCabalFile feature
haskell-pandoc: use new editedCabalFile feature