Add a method to adjust the position of the bubble when the target has changed position

Our application is fairly dynamic and has various pull-out drawers and toolbars. Unfortunately the position of the bubble remains static when drawers are pulled out (this is especially bad when the tour is explaining to the user how to pull the drawer out)

I've added a method to allow us to ask hopscotch to adjust the position of the bubble, once the DOM has changed.

You need to call:
`hopscotch.adjustBubblePosition()`

The implementation is fairly naive. If you have a better way of doing this, please let me know.