Add option for both min/max track image.

Summary:
This is a followup to PR #3850 but now separates min/max track images into different properties.
Closes #4476

Add examples for `minimumTrackTintColor`, `maximumTrackTintColor`, `minimumTrackImage`, `maximumTrackImage` to UIExplorer.
Closes https://github.com/facebook/react-native/pull/4586

Reviewed By: svcscm

Differential Revision: D2779193

Pulled By: nicklockwood

fb-gh-sync-id: 0510a0f496816baacdd0d4be0f3cd3a63a5a9865

Examples/UIExplorer/SliderIOSExample.js

@@ -84,10 +84,38 @@ exports.examples = [
       return <SliderExample step={0.25} />;
     }
   },
+  {
+    title: 'Custom min/max track tint color',
+    render(): ReactElement {
+      return (
+        <SliderExample
+          minimumTrackTintColor={'red'}
+          maximumTrackTintColor={'green'}
+        />
+      );
+    }
+  },
   {
     title: 'Custom thumb image',
     render(): ReactElement {
-      return <SliderExample thumbImage={require('image!uie_thumb_big')} />;
+      return <SliderExample thumbImage={require('./uie_thumb_big.png')} />;
     }
-  }
+  },
+  {
+    title: 'Custom track image',
+    render(): ReactElement {
+      return <SliderExample trackImage={require('./slider.png')} />;
+    }
+  },
+  {
+    title: 'Custom min/max track image',
+    render(): ReactElement {
+      return (
+        <SliderExample
+          minimumTrackImage={require('./slider-left.png')}
+          maximumTrackImage={require('./slider-right.png')}
+        />
+      );
+    }
+  },
 ];