bspwm: add startThroughSession & sessionScript option

Add ability to do a more traditional bspwm startup (using the bspwm-session
script provided by nixpkgs.bspwm) as an alternative to directly starting
sxhkd & bspwm

Also added the ability to specify a custom startup script, instead of
relying on the provided bspwm-session

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

@@ -8,16 +8,39 @@ in
 
 {
   options = {
-    services.xserver.windowManager.bspwm.enable = mkEnableOption "bspwm";
+    services.xserver.windowManager.bspwm = {
+        enable = mkEnableOption "bspwm";
+        startThroughSession = mkOption {
+            type = with types; bool;
+            default = false;
+            description = "
+                Start the window manager through the script defined in 
+                sessionScript. Defaults to the the bspwm-session script
+                provided by bspwm
+            ";
+        };
+        sessionScript = mkOption {
+            default = "${pkgs.bspwm}/bin/bspwm-session";
+            defaultText = "(pkgs.bspwm)/bin/bspwm-session";
+            description = "
+                The start-session script to use. Defaults to the
+                provided bspwm-session script from the bspwm package.
+
+                Does nothing unless `bspwm.startThroughSession` is enabled
+            ";
+        };
+    };
   };
 
   config = mkIf cfg.enable {
     services.xserver.windowManager.session = singleton {
       name = "bspwm";
-      start = "
+      start = if cfg.startThroughSession
+        then cfg.sessionScript
+        else ''
             SXHKD_SHELL=/bin/sh ${pkgs.sxhkd}/bin/sxhkd -f 100 &
             ${pkgs.bspwm}/bin/bspwm
-      ";
+        '';
     };
     environment.systemPackages = [ pkgs.bspwm ];
   };