Add ability to set a container for the backdrop.

This adds the ability to control where the backdrop and overlay are injected into the page.

The backdrop does not behave properly if the target element of the step is in a different [stacking context](https://developer.mozilla.org/en-US/docs/Web/Guide/CSS/Understanding_z_index/The_stacking_context).

One example in bootstrap would be navbar links: http://jsfiddle.net/qpolarbear/8qqpkcgn/5/

This change allows you to place the backdrop inside the same stacking context as the step element.