title: how to display headers in react navigation with TabNavigation
body: <p>I noticed that views in StackNavigation show the header title but if I set those same screens in a TabNavigation it doesn't show a header.  It only shows a header if I wrap a StackNavigation either around each tab, or wrap the TabNavigation nested inside a StackNavigation.</p>
  
  <p>Why don't screens in TabNavigation show a header - is that expected behavior?  If so, is it better to have a StackNavigation in each tab, or one big StackNavigation around the TabNavigation?</p>
  
  <p>// tabs navigation doesn't show a header title in each screen</p>
  
  <pre><code>const TabsNavigator = TabNavigator({
    Home: {
      screen:HomeScreen,
    },
    Profile: {
      screen: ProfileScreen,
    },
  }, {
    tabBarOptions: {
      activeTintColor: '#e91e63',
    },
    navigationOptions: {
      header: {
        visible: true,
      },
    },
  });
  </code></pre>
  
  <p>Header shows when I wrap it in a StackNavigator</p>
  
  <pre><code>default StackNavigator({
    Home: { screen: TabsNavigator },
  });
  </code></pre>
  
  <p>Or is it better to do it this way</p>
  
  <pre><code> export TabsNavigator = TabNavigator({
        Home: {
          screen:StackNavigator({
            Home: { screen: HomeScreen },
          }),
        },
        Profile: {
          screen: StackNavigator({Profile: {screen: ProfileScreen}}),
        },
      }, {
        tabBarOptions: {
          activeTintColor: '#e91e63',
        },
        navigationOptions: {
          header: {
            visible: true,
          },
        },
      });
  </code></pre>