added method to set thumb image

Summary: this change will allow the slider to have different thumb images .

Sets an image for the thumb. It only supports static images
Closes https://github.com/facebook/react-native/pull/3849

Reviewed By: svcscm

Differential Revision: D2665699

Pulled By: nicklockwood

fb-gh-sync-id: 3a767e43170074e2419067d5c8eae61668ebb5e9

Examples/UIExplorer/SliderIOSExample.js

@@ -83,5 +83,11 @@ exports.examples = [
     render(): ReactElement {
       return <SliderExample step={0.25} />;
     }
+  },
+  {
+    title: 'Custom thumb image',
+    render(): ReactElement {
+      return <SliderExample thumbImage={require('image!uie_thumb_big')} />;
+    }
   }
 ];