v3.0.0

Added a bunch of new features and bugfixes:

* Added an option to let the handles behave like pearls on a neckless (see [demo](https://cell303.github.io/react-slider/))
* Added an option to set the minimal distance between handles.
* The handle that is being moved now receives a css class for custom styling
* The handles are now z-indexed, with the most recently moved handle on top.
* Handles are now being moved based on the position where dragging/touching started. This prevents the handle from "jumping" at the beginning and also fixed a bug that occurred with different scroll positions.