dmenu: adding patches to expression

is extended using patches http://tools.suckless.org/dmenu/patches/ this
argument makes it easier to extend dmenu by doing:

dmenu.overwritte {
patches = [
./dmenu-4.4-follow-focus.diff";
];
};

pkgs/applications/misc/dmenu/default.nix

···
-{stdenv, fetchurl, libX11, libXinerama, libXft, zlib}:
+{ stdenv, fetchurl, libX11, libXinerama, libXft, zlib, patches ? null }:
 
 stdenv.mkDerivation rec {
   name = "dmenu-4.6";
···
   };
 
   buildInputs = [ libX11 libXinerama zlib libXft ];
+
+  inherit patches;
 
   postPatch = ''
     sed -ri -e 's!\<(dmenu|stest)\>!'"$out/bin"'/&!g' dmenu_run