Add a --shell option to override the default desktop-shell

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
diff --git a/weston.ini b/weston.ini
index ff0f3ba..a37afe0 100644
--- a/weston.ini
+++ b/weston.ini
@@ -1,5 +1,6 @@
 [core]
-#modules=desktop-shell.so,xwayland.so,cms-colord.so
+#modules=xwayland.so,cms-colord.so
+#shell=desktop-shell.so
 
 [shell]
 background-image=/usr/share/backgrounds/gnome/Aqua.jpg