In some apps this is mostly needed, for instance: when the
CunninghamProvider is nested in the DOM, we want the modal to still
be displayed on top of anything else, then in those cases we will be
able to define a node directly in the body and tell cunningham to
render modals inside it.
We encoutered an issue where stacked modal backdrop were not rendered
above the modal below. It was caused by the dialog element that is
natively rendered on the top layer regardless where it is create in
the DOM. So we decided to use react modal that provides hand crafted
dialog, and implementing a11y features.
Closes#314
