title: React-Navigation conditionals in navigationOptions
body: <p>How would I use an if statement to return a different icon if I have an unread message?
  Or would I not use an if statement? Maybe setting the state on something? Not sure what the best route to take is here.</p>
  
  <pre><code>static navigationOptions = {
  tabBarIcon: ({ focused }) =&gt; (
    focused 
    ? &lt;Icon name="ios-mail" 
      style={styles.activeIconRight} 
      /&gt;
    : &lt;Icon name="ios-mail"
      style={styles.inActiveIconRight}
      /&gt;
  ),
  </code></pre>
  
  <p>}</p>
  
  <pre><code>updateUnread = () =&gt; {
      if (this.state.unread == true) {
        this.props.navigation.setParams({otherParam: 'NEW'})
      } else {
        this.props.navigation.setParams({otherParam: ' '})
      }
    }
  </code></pre>