Align render functions

[thebuilder]

The next major release will align the children and render method, so they both require a ref.
Right now it's confusing if why one is different, and it's hard to remember when using the lib.

It'll still support rendering a React.Node as the children when using the onChange method.

[thebuilder]

Looking at Downshift 2.0 dropping render i think i'll do the same for react-intersection-observer. Nice and simple.
Version 6.0.0 will mark render as deprecated and log a warning, but still accept it.

Up until downshift 2.0.0, we had two props that could be used for your render prop: render and children. Then React released the official context API which uses render props and they called it children. In an effort to develop consistency throughout the ecosystem, we’ve dropped the prop called render and now only support children.
https://blog.kentcdodds.com/downshift-2-released-26b49589e8d2

[thebuilder]

Resolved in #88