allow to configure the placement of the overlay

Due to stacking frames[1], sometimes placing the overlay in the `<body>` just doesn't work (because the element you're trying to highlight is in a higher stacking frame).

This happens a lot if you use, say bootstrap templates, that contains many relative positioned elements. This 2-liner let's you specify where to put the overlay so you can overcome this issue.


[1] http://philipwalton.com/articles/what-no-one-told-you-about-z-index/