Cache policy control for image source

Summary:
In the context of an app an image exists in three resolutions on the server: `thumb` (30px) `feed` (300px) `full` (900px). When looking at an individual item a user can come either from the feed, via a permalink or from other parts of the app. This allows a situation where the `feed` image might or might not already be loaded somewhere in the app. In the detail view I want to render `thumb` with a blur (to quickly display something), then the `feed` image if it exists to have something decent to display until `full` loads. However it is quite a waste to load the `feed` image if it isn't already in cache, and will slow down the time until `full` is loaded. It is possible to track the navigation from feed->detail and that the `feed` image has actually completed loading by the feed component however as component hierarchies grow this turns into quite a lot of prop passing and bad separation of concerns.

NSURLRequests accepts a [Cache Policy](https://developer.apple.com/reference/fo
Closes https://github.com/facebook/react-native/pull/10844

Differential Revision: D4425959

Pulled By: lacker

fbshipit-source-id: 679835439c761a2fc894f56eb6d744c036cf0b49

Examples/UIExplorer/js/ImageExample.js

@@ -304,6 +304,35 @@ exports.examples = [
     },
     platform: 'ios',
   },
+  {
+    title: 'Cache Policy',
+    description: 'First image has never been loaded before and is instructed not to load unless in cache.' +
+    'Placeholder image from above will stay. Second image is the same but forced to load regardless of' +
+    ' local cache state.',
+    render: function () {
+      return (
+        <View style={styles.horizontal}>
+          <Image
+            defaultSource={require('./bunny.png')}
+            source={{
+              uri: smallImage.uri + '?cacheBust=notinCache' + Date.now(),
+              cache: 'only-if-cached'
+            }}
+            style={styles.base}
+          />
+          <Image
+            defaultSource={require('./bunny.png')}
+            source={{
+              uri: smallImage.uri + '?cacheBust=notinCache' + Date.now(),
+              cache: 'reload'
+            }}
+            style={styles.base}
+          />
+        </View>
+      );
+    },
+    platform: 'ios',
+  },
   {
     title: 'Border Color',
     render: function() {