Add optional highlighting of track up to the handle (like #7)

In issue #7 several people requested highlighting the track up to the handle/grabber. 

That was a good idea that I like too, and so I've implemented it as an optional setting (default = off). Styles that fit the current themes are included and adjustable, and I've updated the demo-page. 

I've kept it as DRY as possible, because there was some repetition going on.

This is how it looks when you add `data-slider-highlight="true"` to an input field.
![simple-slider-highlight](https://f.cloud.github.com/assets/3832753/367838/724d4098-a2b3-11e2-8fb6-23a189cf91fb.png)