Delete Item Notification

This is awesome library to show images.

I am missing the one of the extra functionality, which is more helpful for the developers. If you add the delete Notification (delegate). To delete item notification if you can implement this in function just before deleting the item from "initialImages" collection.

Method:
`func headerView(_ headerView: HeaderView, didPressDeleteButton deleteButton: UIButton) {`

Notification/Delegate:
`pageDelegate?.lightboxController(self, willDeleteItem: initialImages[prevIndex], at: prevIndex)`