Allow runtime mod to be customized.

packaging/windows/WindowsLauncher.cs.in

@@ -26,6 +26,7 @@ namespace OpenRA
 		static Process gameProcess;
 
 		// Constants to be replaced by the wrapper / compilation script
+		const string ModID = "MOD_ID";
 		const string DisplayName = "DISPLAY_NAME";
 		const string FaqUrl = "FAQ_URL";
 
@@ -38,6 +39,9 @@ namespace OpenRA
 
 			Directory.SetCurrentDirectory(directory);
 
+			if (!string.IsNullOrEmpty(ModID))
+				args = args.Append("Game.Mod=" + ModID).ToArray();
+
 			var engineArgs = args
 				.Append("Engine.LaunchPath=" + launcherPath)
 				.Select(arg => "\"" + arg + "\"");