Opt-in auto fucus

Current solution when modal automatically searches for tabbable element on open sometimes leads to not desired behavior.

For instance there is a bunch of links inside modal and action button in the end. I don't want to first link be highlighted when user opens a modal.

I think any tabbable element should not be focused by default, default should be focusing on modal div itself with tabIndex='-1' (a11y-focus-scope does it now only if tabbable element can't be found).

And if you want to focus some element, then set option on react-modal2 to enable current mechanic of searching first tabbable element.