title: Create a global stack navigator keeping history
body: <p>I would like to create a global stack navigator callable everywhere in the application, so, imagine a stack navigator :</p>
  
  <pre><code>export default createStackNavigator( //Nav A
      {
          Screen1: Screen1,
          Screen2: Screen2
      }
  </code></pre>
  
  <p>and a second :</p>
  
  <pre><code>export default createStackNavigator( //Nav B
      {
          Screen3: Screen3,
          Screen4: Screen4
      }
  </code></pre>
  
  <p>If i'm on screen2 and trying to navigate to the Screen3, it's working, but I lost the navigation history, in other way, I can't go back to screen2 using the header back button.</p>
  
  <p>How should I handle this ?</p>