Add the friendly navigation setup code from frets-starter

[sirtimbly]

The way of adding top level screens as a list to the props and the actions in frets-starter is pretty clean, but some of it could be brought into the main frets library as an optional module.

Something like:

import {registerNavigation} from "frets"


export interface IRouteKeys {
  About: SampleScreens;
  Home: SampleScreens;
  Users: SampleScreens;
}

export const routeKeys: IKeyObject = {
  Home: "/home",
  Users: "/users",
  About: "/about",
};

F = registerNavigation<MyProps, MyActions>(myApp, routeKeys)