onFocus callback

How do you feel about an `onFocus` callback? So if you have an element that can't receive focus you can add a class yourself or handle the state in something like React and apply a style. I'm trying to build a combobox right now and need to have a fake focus group so the input can remain focused, but still allow choosing a result. Thoughts? I can put together a PR if you like the idea. Maybe something like this?

```
onFocus: options.onFocus || function (member) {
focusNode(member.node)
}
```

Then it would be the user's responsibility to handle focusing the element however they wanted.

_edit_

Realized this can't be solved with just a callback. I'm guessing another option like `groupNode` would be required to determine what the focus group is. Any ideas on that?