Add support for presenting a modal on new coordinator

[ianterrell]

It is possible that the route passed to a new PresentingCoordinator's start method includes a modal element that should be immediately presented.

This commit adds the functionality to do so:

• The scene to present is stored
• Adds helper methods to present the stored scene
• Automatically presents the stored scene from SceneCoordinators

[kent-white]

@ianterrell we haven't ran into this issue before right?

[ianterrell]

@Kent-White don't think so