Get previous hook in chain's filename

[FilipNest]

Allow a hook function to get the filename of the hook function that came before it. Aiming to possibly decouple hooks and modules in the future because of new opportunities opened up by #394 . This will make working with that easier as you'll be able to debug where a hook came from.

Regardless, this is probably a useful debugging function even now.

iris.modules.myModule.registerHook("hook_form_render__login", 3, function(thisHook,data){

    console.log(thisHook.previousHook);

    thisHook.pass(data);

});