add herbstluftwm enable option

nixos/modules/services/x11/window-managers/default.nix

@@ -17,6 +17,7 @@ in
       ./xmonad.nix
       ./i3.nix
       ./xbmc.nix
+      ./herbstluftwm.nix
     ];
 
   options = {

nixos/modules/services/x11/window-managers/herbstluftwm.nix

@@ -0,0 +1,28 @@
+{ config, pkgs, ... }:
+
+with pkgs.lib;
+
+let
+  cfg = config.services.xserver.windowManager.herbstluftwm;
+in
+
+{
+  options = {
+    services.xserver.windowManager.herbstluftwm.enable = mkOption {
+      type = types.bool;
+      default = false;
+      example = true;
+      description = "Enable the herbstluftwm window manager.";
+    };
+  };
+
+  config = mkIf cfg.enable {
+    services.xserver.windowManager.session = singleton {
+      name = "herbstluftwm";
+      start = "
+        ${pkgs.herbstluftwm}/bin/herbstluftwm
+      ";
+    };
+    environment.systemPackages = [ pkgs.herbstluftwm ];
+  };
+}