Add cross-platform Linking module

Summary:
A promise based API for handling Link for Android and iOS. Refer #4971

The iOS part doesn't handle errors. Will need someone with iOS knowledge to do that.

cc skevy ide brentvatne mkonicek vjeux nicklockwood
Closes https://github.com/facebook/react-native/pull/5336

Reviewed By: svcscm

Differential Revision: D2866664

Pulled By: androidtrunkagent

fb-gh-sync-id: 67e68a827e6b85886bfa84e79b897f079e78b1b5

Libraries/react-native/react-native.js

@@ -73,6 +73,7 @@ var ReactNative = {
   get IntentAndroid() { return require('IntentAndroid'); },
   get InteractionManager() { return require('InteractionManager'); },
   get LayoutAnimation() { return require('LayoutAnimation'); },
+  get Linking() { return require('Linking'); },
   get LinkingIOS() { return require('LinkingIOS'); },
   get NetInfo() { return require('NetInfo'); },
   get PanResponder() { return require('PanResponder'); },

Libraries/react-native/react-native.js.flow

@@ -85,6 +85,7 @@ var ReactNative = Object.assign(Object.create(require('React')), {
   IntentAndroid: require('IntentAndroid'),
   InteractionManager: require('InteractionManager'),
   LayoutAnimation: require('LayoutAnimation'),
+  Linking: require('Linking'),
   LinkingIOS: require('LinkingIOS'),
   NetInfo: require('NetInfo'),
   PanResponder: require('PanResponder'),