CLI: Add support for project templates

Summary:
Currently it is not trivial for people to get started with React Native. `react-native init MyApp` just creates a simple app with a single screen. People have to spend time figuring out how to add more screens, or how to accomplish very basic tasks such as rendering a list of data or handling text input.

Let's add an option: `react-native init --template navigation` - this creates a "starter" app which can be easily tweaked into the actual app the person wants to build.

**Test plan (required)**

- Checked that 'react-native init MyApp' still works as before:

<img width="487" alt="screenshot 2017-02-02 16 56 28" src="https://cloud.githubusercontent.com/assets/346214/22559344/b2348ebe-e968-11e6-9032-d1c33216f490.png">

<img width="603" alt="screenshot 2017-02-02 16 58 04" src="https://cloud.githubusercontent.com/assets/346214/22559370/c96a2ca6-e968-11e6-91f7-7afb967920fc.png">

- Ran 'react-native init MyNavApp --template'. This prints the available templates:

```
$ react-native init MyNavApp
Closes https://github.com/facebook/react-native/pull/12170

Differential Revision: D4516241

Pulled By: mkonicek

fbshipit-source-id: 8ac081157919872e92947ed64ea64fb48078614d

react-native-cli/index.js

@@ -48,6 +48,7 @@ var semver = require('semver');
  *   if you are in a RN app folder
  * init - to create a new project and npm install it
  *   --verbose - to print logs while init
+ *   --template - name of the template to use, e.g. --template navigation
  *   --version <alternative react-native package> - override default (https://registry.npmjs.org/react-native@latest),
  *      package to install, examples:
  *     - "0.22.0-rc1" - A new app will be created using a specific version of React Native from npm repo
@@ -129,7 +130,8 @@ if (cli) {
       '  Options:',
       '',
       '    -h, --help    output usage information',
-      '    -v, --version output the version number',
+      '    -v, --version use a specific version of React Native',
+      '    --template use an app template. Use --template to see available templates.',
       '',
     ].join('\n'));
     process.exit(0);
@@ -264,8 +266,7 @@ function getInstallPackage(rnPackage) {
 }
 
 function run(root, projectName, options) {
-  // E.g. '0.38' or '/path/to/archive.tgz'
-  const rnPackage = options.version;
+  const rnPackage = options.version; // e.g. '0.38' or '/path/to/archive.tgz'
   const forceNpmClient = options.npm;
   const yarnVersion = (!forceNpmClient) && getYarnVersionIfAvailable();
   var installCommand;