Passes element to onComplete callback.

It gives us two things:
1. Performance boost if we want to perform actions on the element (as we don't need to search for that element again).
2. Ability to perform actions on individual elements if we have several galleries on the page (the onComplete call is called for each gallery).


I was missing this functionality so I went ahead and implemented it. Ping me, If you need examples where this could be applied.