Add CSS Transition to Slider Change

I've used the [Frequency Decoder fd-slider](https://github.com/freqdec/fd-slider) for years and one of the things I always liked was that if you click/tap on a space on the slider (other than the slider handle) then the slider would animate/transition smoothly to that new position.

This Pull Request adds SCSS and JS to *rangeslider.js* to optionally add a CSS transition effect like described above. I set the defaults to enable the transition, but that is a judgment call. You may wish to reverse that (which will be super-simple with how I've coded it).

I like what I've see of rangeslider.js!  I appreciate that it is very lightweight and efficient, and does what it does well.  :)  And since [fd-slider](https://github.com/freqdec/fd-slider) is no longer being updated I'm hoping to switch over to rangeslider.js at some point in the future.

Thanks!
Sean