title: How to open Drawer When use a custom icon in Scene
body: <p>I using Drawer my app, my problem is i want use a custom icon in Drawer Scene and its ok but I dont see any docs to tell me how to open or close the drawer!</p>
  
  <p>This is my router :</p>
  
  <pre><code>&lt;Scene
      key="TabbarWrapper"
      component={NavigationDrawer}
      initial={true} &gt;
      &lt;Scene key="Tabbar"
             tabs={true}
             initial={true}
             tabBarStyle={styles.tabBar}
             default="ProductExplorer"&gt;
  
          &lt;Scene key="ProductExplorer"
                 title="Lists"
                 icon={TabIcon}
                 iconName={"list"}
                 initial={true}
                 leftButtonIconStyle={{tintColor: "#FFF"}}
                 renderBackButton={backButtonFunction}
                 component={ProductExplorer}/&gt;
  
          &lt;Scene key="Profile"
                 title="Profile"
                 icon={TabIcon}
                 iconName={"gear"}
                 leftButtonIconStyle={{tintColor: "#FFF"}}
                 renderBackButton={backButtonFunction}
                 component={Profile}/&gt;
      &lt;/Scene&gt;
  
  &lt;/Scene&gt;
  </code></pre>
  
  <p>and this is my button renderer function :</p>
  
  <pre><code>let backButtonFunction = function () {
      var TouchableElement = TouchableHighlight;
      if (Platform.OS === 'android')TouchableElement = TouchableNativeFeedback;
      return (
          &lt;TouchableElement onPress={()=&gt;{ 
             **** my problem is here **** what do you think i should do?!what function must be call here to toggle open and close menu?
           }} style={{position : "absolute",left: 12,bottom:12}}&gt;
              &lt;Icon style={{color: "#FFF"}} name={"bars"} size={23}/&gt;
          &lt;/TouchableElement&gt;
      );
  };
  </code></pre>