Managing focus

Looking at the demos it looks like there is some focus management missing on the modals. Here's what needs to be added to fix it:

* When a modal is open, focus should be sent to the modal (either on the element itself via .focus() and tabindex="-1" or sent to the first interactive element.

* When a modal is open, focus should be trapped inside the modal (so you can't access elements behind it when tabbing with the keyboard).

* When a modal is open, pressing the escape key should close the modal.

* It would be nice if the modals had an explicit close button with an accessible name (e.g. `X`.

* When a modal is closed, focus should be send back to the element which opened it.

An example of an accessible modal:
http://edenspiekermann.github.io/a11y-dialog/example/

The WAI-ARIA authoring practices guide on modals:
https://www.w3.org/TR/wai-aria-practices/examples/dialog-modal/dialog.html