Custom navigation buttons outside & with correct disabled state

As described in #2013, it's possible to render navigation buttons outside of the slider component by passing the Slider ref and calling the respective methods on it (`slickNext()` and `slickPrev()`).

However, I didn't find a way to correctly disable the buttons based on the current slider state.
Maybe some additional properties like `hasNext` or `hasPrev` should be exposed on the Slider class?