Feature Request: differentiate "active" and "inactive" controls

To improve the meaningfulness of the 'js_prev' and 'js_next' controls, one should be able to style them differently if they are "active" or not, that is, if clicking them will actually move the slider or not.

For example, the 'js_prev' control does nothing when the slider is at the start position, but the 'js_next' will move the slider, and you may want to show a dull grey left chevron and a bold black right chevron. There should be something such as an 'active' or 'inactive' class (or both) that get added and removed from 'js_{prev,next}' as needed, so that this kind of styling is possible.