Add <Text> shadow support

Summary:
Add three new TextStylePropTypes for \<Text>
- textShadowOffset
- textShadowRadius
- textShadowColor
Closes https://github.com/facebook/react-native/pull/4975

Reviewed By: svcscm

Differential Revision: D2796278

Pulled By: nicklockwood

fb-gh-sync-id: f8c3fa210e664428b029b9fba8eca4a8eb81c08d

Examples/UIExplorer/TextExample.ios.js

@@ -424,6 +424,17 @@ exports.examples = [
       </View>
     );
   },
+}, {
+  title: 'Text shadow',
+  render: function() {
+    return (
+      <View>
+        <Text style={{fontSize: 20, textShadowOffset: {width: 2, height: 2}, textShadowRadius: 1, textShadowColor: '#00cccc'}}>
+          Demo text shadow
+        </Text>
+      </View>
+    );
+  },
 }];
 
 var styles = StyleSheet.create({