Provide an innerRef prop

[nbouscal]

This allows access to the underlying DOM wrapper node, which is useful if you need to call focus. In my use case I have several different HotKeys components on different parts of the page, and want to pass focus from one to another. They don't always have children with a tabindex, so there is no other element to add a ref to. Another option could be using state and the focused property, but in my testing the focused property doesn't really seem to work.

The innerRef pattern seems to be idiomatic, being used by e.g. react-router and formik.

Another option would be using ref forwarding, but that would be backwards-incompatible for anyone currently using refs to access the HotKeys component itself.

[greena13]

I had to modify to match the new architecture (and add a sanity test), but the idea is now in master and will go out with the next release.