title: React native react-navigation tabBarIcon does not display
body: <p>I'm having a TabNavigator where I want to have icons at each tab together with a label. However, even though I've tried numerous ways in order to get the icon to appear, nothing happens.</p>
  
  <pre><code>// Imports...
  
  const StartScreen =  TabNavigator({
      Home: {
          screen: HomeTab,
          navigationOptions: {
              tabBarLabel: 'Test',
              tabBarIcon:() =&gt; &lt;Icon size={ 20 } name={ 'cogs' } color={ 'red' }/&gt;
          }
      },
      Calendar: {
          screen: CalendarTab,
          navigationOptions: {}
      }
  });
  
  StartScreen.navigationOptions = {
      title: 'TestApp',
      headerTintColor: '#ffa500',
      showIcon: true
  };
  
  export default StartScreen;
  </code></pre>
  
  <p>And yes, I've tried out using the Icon-component so I know that it works.</p>
  
  <p>Any tips or guidance would be really helpful, thanks!</p>