Add hook-ey react-intersection-observer API

[lemonmade]

This PR adds a new useIntersection hook to react-intersection-observer, and slightly reworks the API of the component version to feel more closely related. The README has all of the goods, but basically, the hook version just gives you the state + a ref to throw on a DOM node, so it's quite simple to use the hook version:

function MyComponent() {
  const [intersection, intersectionRef] = useIntersection();

  return (
    <div ref={intersectionRef}>Intersection: {intersection.isIntersecting}</div>
  );
}

[cartogram]

this makes sense as a shared hook https://github.com/Shopify/quilt/pull/609/files#diff-99a7b9f41d8c46ef5eb67c2b0da03b2aR3

[marutypes]

agreed

[lemonmade]

Yes, agreed, we just don't have a place for them yet, and I didn't want to throw that in here too.

[marutypes]

Some small (mostly documentation readability) comments, but this looks good overall to me.

[marutypes]

should each of these maybe be clones instead of a reference to the same object?

[lemonmade]

I don't think it matters, all the fields are meant to be readonly