title: Scroll to top of current tab when it is pressed again in TabNavigator in React Native
body: <p>I have react native app with <code>BottomTabNavigator</code>. In this if the current tab selected has been scrolled down <code>FlatList</code> and then press on next tab in the navigator and again navigate back to previous tab the scroll is still on that position i.e., at down. How to scroll to top of the current tab after navigation?</p>
  
  <p>What i have is:</p>
  
  <p>navigation.js</p>
  
  <pre><code> import {createBottomTabNavigator} from "react-navigation";
       .....................................
  
      name: {
      screen: createBottomTabNavigator({
        first: { screen: firstNavigation },
        second: { screen: secondNavigation },
        third: { screen: thirdNavigation }
      })
    } 
  </code></pre>
  
  <p>i have three tabs on the bottom named first,second and third, navigating from first to second and the faltlist in second tab have this issue.</p>
  
  <p>second.js</p>
  
  <pre><code>        &lt;View&gt;
            &lt;FlatList
              data={this.causes}
              keyExtractor={this._keyExtractor}
              renderItem={this._onPress}
              contentContainerStyle={styles.flatListContainer}
            /&gt;
          &lt;/View&gt;
  </code></pre>