[Carousel style 2] Added an option to toggle thumbnail controls on medium-and-above screens.

**Done:**
* Added CSS to hide the `item n of x` controls and display the thumbnail controls when class `show-thumbs` is added to carousel style 2.
* Updated the working examples to include an example of the carousel style 2 with thumbnails.

**To do:**
* Style the thumbnails so that they appear at the correct size and in the correct location.
* Update the documentation.

cc/@zachfalsetto