title: does the tabview renders components on swipe only?
body: <p>The screen i am working on needs some tab navigation . its working great but there is one problem. It renders the component on swipe only when i press on the tab i want to go to . it does not do anything.
  Ex: I am on tab number 3 and i want to go to tab 1. 
  If i click on tab 1 , nothing happens . i have to swipe to it passing tab 2 in the process .
  I don't want that . Here's my code.</p>
  
  <pre><code>&lt;TabView
     navigationState={this.state}
     renderScene={this.renderScene}
     onIndexChange={index =&gt; this.setState({ index })}
     initialLayout={{ width: Dimensions.get('window').width }}
   /&gt;
  </code></pre>
  
  <p>render scene function</p>
  
  <pre><code>renderScene = ({ route, jumpTo }) =&gt; {
      switch (route.key) {
        case 'activity':
          return &lt;ProfileActivity jumpTo={jumpTo} /&gt;;
        case 'circles':
          return &lt;ProfileCircles jumpTo={jumpTo} /&gt;;
        case 'friends':
          return &lt;ProfileFriends jumpTo={jumpTo} /&gt;;
        default:
          return null;
      }
    };
  </code></pre>
  
  <p>state</p>
  
  <pre><code>  state = {
      index: 0,
      routes: [
        { key: 'activity', title: 'Activity' },
        { key: 'circles', title: 'Circles' },
        { key: 'friends', title: 'Friends' }
      ]
    };
  </code></pre>