[xmonad] darcs patch: Script callbacks for hooks

Hi Everyone, I've added the ability to call a script with an argument to direct execution. For example, if you'd like to run a hook on startup that does something like manage your dzen2 instance, all you would need to do is make sure that:

execScriptHook "startup"

was somewhere in your startup hook definition, and the script ~/.xmonad/hooks would be called with a single argument of "startup". --trevor