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.
Why are you doing this (here and below)? `mountReactComponents` is already a function…