Update NouisliderProps.tooltips typing to match noUiSlider docs

[wjhurley]

This update changes the typings for NouisliderProps.tooltips to match the noUiSlider docs. I have tested these typings against nouislider-react ^3.3.5 to ensure they pass TSlint checks and that there are no compile-time or run-time errors. I tested the Formatter type using wnumb ^1.2.0 as well as a custom function that returned to and from methods.

I realized after submitting the initial pull request that the types in index.js needed to be updated as well as the github.io page. I've updated those files as well and ran yarn test, which passed all tests.

[mmarkelov]

@wjhurley lgtm. Thanks!