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
dd9b3e13a9