Added react-native run-ios

Summary:
Works the same way as `react-native run-android`, but targets iOS simulator instead. Under the hood, it uses `xcodebuild` to compile the app and store it in `ios/build` folder, then triggers `instruments` and `simctl` to install and launch the app on simulator.

Since Facebook relies on BUCK to build and run iOS app, we probably won't use `run-ios` internally. That's why I'm putting this as public PR instead of internal diff.

To test this, I hacked global `react-native` script to install react native from my local checkout instead of from npm, cd into the folder and ran `react-native run-ios`.
Closes https://github.com/facebook/react-native/pull/5119

Reviewed By: svcscm

Differential Revision: D2805199

Pulled By: frantic

fb-gh-sync-id: 423a45ba885cb5e48a16ac22095d757d8cca7e37

local-cli/cli.js

@@ -25,6 +25,7 @@ var link = require('./library/link');
 var path = require('path');
 var Promise = require('promise');
 var runAndroid = require('./runAndroid/runAndroid');
+var runIOS = require('./runIOS/runIOS');
 var server = require('./server/server');
 var TerminalAdapter = require('yeoman-environment/lib/adapter.js');
 var yeoman = require('yeoman-environment');
@@ -46,6 +47,7 @@ var documentedCommands = {
   'link': [link, 'Adds a third-party library to your project. Example: react-native link awesome-camera'],
   'android': [generateWrapper, 'generates an Android project for your app'],
   'run-android': [runAndroid, 'builds your app and starts it on a connected Android emulator or device'],
+  'run-ios': [runIOS, 'builds your app and starts it on iOS simulator'],
   'upgrade': [upgrade, 'upgrade your app\'s template files to the latest version; run this after ' +
                        'updating the react-native version in your package.json and running npm install']
 };