Update example app for new develop-0.2 API

[AndrewSB]

I'm trying to update to the latest cordux API and I'm having trouble understanding what the started in

public func coordinatorForTag(_ tag: String) -> (coordinator: AnyCoordinator, started: Bool)?

does. Does that mean my SceneCoordinator should be holding on to instances of it's child coordinators, and know whether or not they've been presented before, i.e. started?

Being able to see how the new API is used in the example app would be awesome 👍