[Portal] - Portal Component should be Optional

[mlaursen]

Right now there is no way to allow the portal to be created in the current tree except when server side rendered.

Related tickets:

• [Portal] - Portal should not use the undocumented unstable_renderSubtreeIntoContainer #159
• Portal should be able to animate like CSSTransitionGroup #166
• Nested Dialogs don't display correctly #229