Add sliderRedraw callback

I'm not sure if you'd like to add this callback into the master but it was very useful to me given the following circumstance:

**The story:**

* I have a slider of *n* slides
* It is fully responsive (no max height or width)
* Each slide has an image or video that takes up the full width and height of the slide
* Each slide has live text on top of the video or image. This text is absolutely positioned.

**The problem:**

How do I vertically postion text within a dynamic height and width wrapper?

**The idea:**
Using the `display: table; height: 100%` and 'display: table-cell' I should be able to vertically position my text within a variable height slide. However, for this to work, the parent element needs to have a height set to it and `height: 100%;` breaks the slider height calculations.

**The solution:**
Using the `onSliderRedraw()` callback, I can append the calculated height for the bxslider-viewport to my text wrapper:

    $( '.feature-slider ul' )bxSlider({
      onSliderRedraw : function(  $sliderViewport, width, height ) {
        $sliderViewport.find( '.feature-intro-wrapper' ).css 'height', height
      }
    });