title: Modal window in React Native with react-navigation
body: <p>I am using <code>react-navigation</code> in React Native and I want, on startup, to determine if the user is logged in, and if he/she is already logged in, I want to open a modal window (full screen).</p>
  
  <p>How is this best done? I cannot find anything in the react-navigation docs where a screen is conditionally shown.</p>