Support manual pin/unpin

[AndersDJohnson]

Add a prop to manually set pin/unpin state, e.g. to enable a hide button that unpins when pinned if user wants to hide headroom without scrolling down.

[denkristoffer]

You can set a reference and use this.ref.pin() or this.ref.unpin().

[DocCaliban]

Going to agree with @AndersDJohnson and point out that using a ref is discouraged.
https://reactjs.org/docs/refs-and-the-dom.html#dont-overuse-refs

[barrettluke]

translateY for unpin is HARDCODED '-100%' please change this.