…config location (#460)

Summary:
**Summary**

Added an option `extendsBabelConfigPath` for extending a custom babel preset.

**Test plan**

`metro.config.js`
```js
module.exports = {
  transformer: {
     babelTransformerPath: require.resolve('./transformer.js'),
  },
}
```

`transformer.js`
```js
const transformer = require('metro-react-native-babel-transformer');

module.exports.getCacheKey = transformer.getCacheKey;

module.exports.transform = function({ filename, options, src, plugins }) {
  return transformer.transform({
    filename,
    options: Object.assign(options, { extendsBabelConfigPath: require.resolve('babel-preset-cool-custom-thing') }),
    src,
    plugins
  });
};
```
Pull Request resolved: #460

Differential Revision: D18201840

Pulled By: cpojer

fbshipit-source-id: a729e8be77803762211abe52ce13aa7d0b690dd2