Add basic example of a custom update loop

[easynam]

There currently aren't any examples of how to do this aside from the actual implementations in schedule_runner and the bevy_winit create, and quite a few people have asked about it on the discord.

This adds a basic example that reads input from stdin and updates the app once for each line.

[Plecra]

Might a bevy::app::Events be a more appropriate way to present the user input?

[cart]

@Plecra I think for a more serious implementation events might be better, but i kind of like the minimalism of this example

I dig this. I think its good to go after a rebase (sorry about that ... i accidentally broke builds on the master branch. but its fixed now).