handle multiple css classes for previous/next links

For Orbit module, whenever more than 1 css class is defined for `next_class` and `prev_class`, the onclik event is not triggered.

Example: you define the options as follow: `data-options="next_class:orbit-next-button button;"`. The selector to get the element is: `.orbit-next-button button` instead of `.orbit-next-button.button`