title: React Native Navigation: Only show tab label for focused item
body: <p>I'm just trying to conditionally show/hide the label for the Tab navigator's items based on if they are focused or not. I am able to change the icon's tint color like so:</p>
  
  <pre><code>tabBarIcon: ({ focused }) =&gt; {
     const icon = &lt;Image
       style={{
          width: 25,
          height: 25,
          tintColor: focused ? colors.primary : colors.inactive
       }}
       source={require('./assets/account.png');}
       /&gt; ;
  
       return icon
  </code></pre>
  
  <p>But trying to change the showLabel boolean conditionally based on the same prop isn't working?</p>
  
  <pre><code>tabBarOptions={{
         activeTintColor: colors.primary,
         inactiveTintColor: colors.inactive,
         showLabel: ({ focused }) =&gt; {
           return focused ? true : false
         },
  </code></pre>
  
  <p>The labels appear for all items on the Tab bar. </p>
  
  <p>Any/all help appreciated!</p>