add a method for specifying the parent element instead of window

[davidmaxwaterman]

The core-overlay seems to assume that it should overlay the entire window. In my application, we want it to only overlay one part of the window (it's essentially split into two parts, each is a different application and the dialog only applies to one of them).

Please consider adding an attribute so that the user can specify the parent element so that the target is 'copied' into that instead of the main body, and it 'grey out' only that element's extent and center the target dialog in that area (vertically and horizontally, of course).

Ref: googlearchive/paper-dialog#57