Expose mount/unmount and allow them to be scoped by a selector.

Looking for comments on this before I attempt to add tests.

I'm currently using this for an app which has only a few react components, including some that are not present on page load and may disappear later, so I need to mount/unmount components from parts of the page manually in other javascript.
It is a function, but it's not a proper event handler. If I don't wrap it like this, the event gets passed in as the `rootNode`.