Expose jQuery UI slider events for RangeSlider

I'd like to be able to hook into events related to the jQUI slider and make a decision whether now is the best time to update the UI (I have a large amount of data so I don't want to refresh the UI too much).

I went about subclassing `RangeSlider` for my project but didn't want to rewrite or *copy* the jQuery UI slider bindings, this PR allows subclasses or once off instances to customise how the slider interacts with the graph.