Add callback after card is rendered/appended by editor

A callback such as `onTransition` or `onAppend` (or perhaps `didRender`) could be a useful hook for cards that are rendered by the editor. A use case is for cards that rely on a 3rd-party to render completely, such as a facebook-related card that needs to call `window.FB.XFBML.parse()` after its element is appended.
