Added before and after event

[teawithfruit]

To handle the content before and after sliding, I've added two events which can be used with the following code.

Reveal.addEventListener( 'beforeslide', function( event ) {
  console.log(event);
});

Reveal.addEventListener( 'afterslide', function( event ) {
  console.log(event);
});

[hakimel]

This is a useful addition.

There shouldn't be any need to dispatch these events from the individual "navigate" methods though, right? Those methods all end up calling slide(), and you're already dispatching the same events from within the slide() function.