Added 'captionsAttr' options in order customize image caption value

[enrico-sorcinelli]

This simple PR adds captionsAttr option in order to customize img attribute from which to get the caption value. Default value is title (that is, as the current behaviour).
For example:

JS

$('.bxslider').bxSlider({
   mode: 'fade',
   captions: true,
   captionsAttr: 'data-caption'
});

HTML

<ul class="bxslider">
  <li><img src="/images/730_200/tree_root.jpg" title="Funky roots" data-caption="Source: Thinkstock" /></li>
  <li><img src="/images/730_200/hill_road.jpg" title="The long and winding road" data-caption="Source: Flickr" /></li>
  <li><img src="/images/730_200/trees.jpg" title="Happy trees" data-caption="Source: Getty Images" /></li>
</ul>