Add callbacks

[fijiwebdesign]

Will add a callback to be fired on each element that is highlighted.

$('body').highlight('example text', {className: 'highlight'}, function(el) {
      // el is the current highlighted element
      $(el).on('hover', function() { console.log('el', el) })
    });

[knownasilya]

Thanks! Could you document this new feature in the readme?

[fijiwebdesign]

@knownasilya Added to readme.