Add keyboardAppearance prop to TextInput component.

Summary: Resolves #3649.
Closes https://github.com/facebook/react-native/pull/4012

Reviewed By: javache

Differential Revision: D2636538

Pulled By: nicklockwood

fb-gh-sync-id: 022e79d8f8fa684cad43af1a51f728d60ac652a8

Libraries/Components/TextInput/TextInput.js

@@ -161,6 +161,15 @@ var TextInput = React.createClass({
       'twitter',
       'web-search',
     ]),
+    /**
+     * Determines the color of the keyboard.
+     * @platform ios
+     */
+    keyboardAppearance: PropTypes.oneOf([
+      'default',
+      'light',
+      'dark',
+    ]),
     /**
      * Determines how the return key should look.
      * @platform ios