title: React Navigation: nesting stack and tab navigator
body: <p>I have a <code>TabNavigator</code> inside a <code>StackNavigator</code>. I cannot define headers inside the different classes that are within the Tabs themself as the <code>TabNavigator</code> has no header (that I know of). So I tried to define the it when initializing the <code>StackNavigator</code>. A button appears on top, but pressing it gives an error for redirecting to an undefined place. probably the <code>this.props.navigator.navigate</code> part contains the mistake, with the <code>this</code> not referring to what I want it to refer to, but I can't find the right syntax anywhere.</p>
  
  <pre><code>const NestedNavigator = createBottomTabNavigator ({
    "route1" : {screen : Screen1,
    },
    "route2" : Screen2,
    "route3" : Screen3
    }, 
  );
  
  const Navigator = createStackNavigator ({
    "routeA" : ScreenA,
    "routeB" : {
      screen : NestedNavigator, 
      navigationOptions: { headerRight: (&lt;Button title="home" onPress={() =&gt; this.props.navigation.navigate("routeC")}/&gt;)}},
    "routeC" : ScreenB},
    {
      initialRouteName: "routeA",
    }
  );
  </code></pre>