Add accessibilityElementsHidden prop

Summary:
Allow iOS to have similar accessibility functionality to Android. This PR exposes the `accessibilityElementsHidden` property on iOS which is similar to Android's `importantForAccessibility="no-hide-descendants"`

I didn't see any existing examples for testing native props being passed through, but I did add an example to the RNTester app. I've attached some screenshots using the Accessibility Inspector to verify the property was correctly passed through.

![a](https://user-images.githubusercontent.com/603528/34998153-50e66776-faac-11e7-826d-1445a6813929.png)
![b](https://user-images.githubusercontent.com/603528/34998158-535a7420-faac-11e7-80d4-992fb7cd82dd.png)

I've updated the website with appropriate documentation.

facebook/react-native-website#141

![screen shot 2018-01-16 at 10 23 50](https://user-images.githubusercontent.com/603528/34998202-6f2f39a6-faac-11e7-8651-0cfe8e037a30.png)
![screen shot 2018-01-16 at 10 23 59](https://user-images.githubusercontent.com/603528/34998205-711d6f94-faac-11e7-974d-54340c72fce4.png)

[IOS] [FEATURE] [View] - Added accessibilityElementsHidden property
Closes facebook/react-native#17627

Differential Revision: D6806444

Pulled By: hramos

fbshipit-source-id: 50d31fdb92f4c59ae9355b019c422418b2e6cc24

js/AccessibilityIOSExample.js

@@ -55,6 +55,9 @@ class AccessibilityIOSExample extends React.Component<{}> {
           accessible={true}>
           This text component's accessibilityLabel is set explicitly.
         </Text>
+        <View accessibilityElementsHidden={true}>
+          <Text>This view's children are hidden from the accessibility tree</Text>
+        </View>
       </View>
     );
   }