Make the example even more real-world

[ericvicenti]

Make this codebase a bit larger, demonstrate other navigation actions, and maybe build a basic custom navigator with a fade animation.

[jp928]

@ericvicenti I would like to contribute to make more complicated transition animation. However, I couldn’t make a working customed tranaitioner. Or it can be achieved by pure css. Is that possible to brief clarify what would be a best practice with React Navigation for web?