feat(offcanvas): Added Offcanvas submenu functionality.

I am not sure about the unit tests, as this is the first time I ever do it, but I hope it's a cool addition overall.
Also, I've bumped the Foundation CSS version to 5.4.6 (for the submenu CSS).
Why the change so we only listen for clicks on `<li>` elements? The offcanvas could contain non-list content which would cause `offCanvasWrap.hide()` to not fire.