Support Popper v2 virtual element

Created by: septatrix

Dropdowns accept a reference option for Popper however the type is restricted to string|element.

Popper itself on the other hands also allows objects which provide a getBoundingClientRect method. This can be quite useful when one wants implement custom context menus, arbitrary popup location on maps/images or otherwise does not have an actual DOM element as a reference but instead only raw coordinates.

The only thing required should be to also accept object (or only object which have getBoundingClientRect set).