Custom navigation buttons outside & with correct disabled state

[simonhenke]

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?