Keep element visible once it has been visible once

Hello,

Firstly, thank you for this component, we've really enjoyed using it!

Often when using it we find we have to wrap the component and use `onChange` to store some local state to keep track of if the component has been visible once. This is so we keep the component on the page constantly once it's been seen by the user once (so if they are scrolling up/down a big list, things don't pop in and out).

This is pretty easily done by storing state and listening to `onChange`, but I was wondering if you'd consider a PR that added a `keepVisibleOnceSeen` prop that would do this out of the box. It would remove a small amount of boilerplate we have to have each time we use this component.

If you don't then that's totally cool, but if you think this is a reasonable addition I'd be happy to make the PR and do the work.

Thanks!

Jack