Before/after move callback enhancements

This builds off my last pull request (#104) to also pass the next element into the callbacks after the index.

Nothing _should_ break, since it's an extra argument at the end (yay javascript)

This is useful for preparing the next screen to be shown, where you may wish to select elements inside that section. Doing that with the index would require more code, plus it's already done in `onepage-scroll`, just not currently passed in.

I suppose the correct element could be selected by its `.selected` class with jQuery, it's just nice if it's already there.