Add support for React refs on `Box`

[DannyDelott]

This is a great library, however it is missing a way to attach a ref to Box's underlying DOM node. This is useful for integrations with other third party libraries.

Specifically we want to be able to integrate with react-beautiful-dnd, which requires an exposed ref on the draggable items.

Would you be open to a PR that introduces an elementRef prop to the Box component?

Example usage:

render() {
  return (
    <Box elementRef={...}>
      {this.props.children}
    </Box>
  );
}

[robinweser]

Hey! Yeah sure, go for it! I‘d prefer an update to latest React using the forwardRef API though. If you need any help, just ping me!

[DannyDelott]

The forwardRef API won't work for ScrollView, because it currently exposes instance methods via ref, ie: scrollTo, getScrollPosition, et al. These would become inaccessible with a forwardRef refactor.

Let me know what your thoughts, and if you'd be willing to accept the elementRef as proposed here.

[robinweser]

Yeah that makes sense! innerRef oder elementRef are common names I guess. :)