Added configuration option for custom carousel items selectors

I've added a new configuration option "itemsSelector", which makes it possible to use other elements as carousel items, than the direct childrens of the carousel selector. Useful as example in the Drupal context, where you have a lot of additional html markup and wrappers.

Example simple Carousel Configuration:

      multiteaserCarousel.owlCarousel({
        singleItem: true,
        itemsSelector: '.example-wrapper-inside-carousel .another-wrapper > .we-want-this-as-items'
      });