Allow rn-cli.config.js to specify the default transformer

Summary:
This will allow consumers to supply their own transformer to all `react-native` cli commands by simply implementing `rn-cli.config.js` and overriding `getTransformModulePath()`. That way they don't have to fork various parts of the iOS and Android build system that React Native already provides just to add a `--transformer` command line argument.

**Test plan:** Applied this patch to the React Native version in my app, implemented `getTransformModulePath()` in my `rn-cli.config.js`, and verified that my custom transformer is invoked.
Closes https://github.com/facebook/react-native/pull/7961

Differential Revision: D3404201

Pulled By: foghina

fbshipit-source-id: c7eaa85de84d485d06d23a2ffea899821b2cf71c

local-cli/bundle/bundleCommandLineArgs.js

@@ -22,7 +22,7 @@ module.exports = [
     command: 'transformer',
     description: 'Specify a custom transformer to be used',
     type: 'string',
-    default: require.resolve('../../packager/transformer'),
+    default: null,
   }, {
     command: 'dev',
     description: 'If false, warnings are disabled and the bundle is minified',