Add a mechanism that communicates actual positioning strategy used by popup

[ccjoel]

My team is in need of some mechanism to verify the actual positioning strategy of the popup (not the suggested one passed as prop, since the popup nicely repositions if the popup would be off screen).

We are thinking that we could add a class to the tw-popup container. It would specify the actual strategy used upon running the strategy function. This way, we could modify our CSS to move around an element (say, a caret for a tooltip) as the popup repositions.

I will definitely try to implement and submit a PR if this is a good solution to our problem. Let me know if there's an alternate approach you guys suggest. You would be merging the PR, after all.