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

rn-cli.config.js

@@ -33,4 +33,9 @@ module.exports = {
       return [path.resolve(__dirname, '..')];
     }
   },
+
+  getTransformModulePath() {
+    return require.resolve('./transformer');
+  },
+
 };