Popover, initial/default open state

[grumpyoldman-io]

Hey there, we were looking at how to have a popover element in our application, but in some cases we need it to be open on render. We've identified some ways to do this:

• use the static prop, and completely manage our own state (basically duplicate the entire component state and listeners)
• use a ref, and fire a click element on the trigger button manually (hacky, as we go from react -> DOM -> react)

ideally there would be a way to provide initial state to the Popover via a prop like:

<Popover defaultOpen={true} />

or the likes.