Controlled component

Hi @jaredpalmer, first of all thank you for your work!

I'm trying to use it as a Controlled Component but I think that I'm missing something(https://codesandbox.io/s/104l1rp7j4).

I though that by passing a prop `isOpen` we can control its state.

Start by reproducing the controlled-component behavior in the linked CodeSandbox and review the component's public API for the `isOpen` prop. Confirm whether the prop is intended to control the accordion state, then verify that the component responds to external open-state changes and document or test the expected behavior.

Written by the indexing model from the issue text.