Add onOverlayClicked callback

[rettrich]

I have a use case where I'd like to differentiate between closing the tour by dismissing via the close button and clicking on the backdrop overlay - therefore I would like to propose a feature request to add a click callback that is triggered when the backdrop overlay is clicked during an active tour.

It could be added to the Config and look similar to the other hooks:

type Config = {
  // ...
  // triggered when clicking on the backdrop overlay
  onOverlayClicked?: (element?: Element, step: DriveStep, options: { config: Config; state: State }) => void;
}