title: Pass props to underlying screens
body: <p>I have the following nested structur: </p>
  
  <pre><code>- StackNavigator
   - TabBar
     - TabScreen1
     - TabScreen2
     - TabScreen3
     - TabScreen4
   - NormalScreen
  </code></pre>
  
  <p>The main stacknavigator is called like this: </p>
  
  <pre><code>&lt;Router notifications={6} /&gt;
  </code></pre>
  
  <p>And looks like this: </p>
  
  <pre><code>    const Router = StackNavigator({
    MainTabBar: { screen: MainTabBar },
    ChartDetailScreen: { screen: ChartDetailScreen },
  }, {
    headerMode: 'none',
    initialRouteName: 'MainTabBar',
  });
  </code></pre>
  
  <p>In one of these TabScreens I need to access the <code>notifications</code> property, which is passed to the stacknavigator.</p>
  
  <p>The screens configuration basically looks like this :</p>
  
  <pre><code>Screen1: {
        screen: Screen1,
        navigationOptions: {
          tabBarLabel: 'Screen1',
          tabBarIcon: ({ tintColor }) =&gt; (
            &lt;Image ... /&gt;), &lt;-- I need notifications here!
        },
      },
  </code></pre>
  
  <p>How do I pass the <code>notification</code> prop of the outer stacknavigator to this/all screens? I tried to access <code>screenProps</code> with <code>navigationOptions: (screenProps) =&gt; {...}</code>, but there didn't appear this property.</p>
  
  <p>I use React Navigation <code>"version": "1.0.0-beta.19"</code>.</p>