Should use _.merge in place of _.assign

[ipsips]

https://github.com/sumul/fractal-react-adapter/blob/master/src/adapter.js#L99

Otherwise it's not possible to specify custom plugins via babelConfig, eg:

const reactAdapter = require('fractal-react-adapter')({
  babelConfig: {
    extensions: ['.jsx'],
    presets: ['es2015', 'es2017', 'stage-3', 'react'],
    plugins: ['transform-class-properties']
  }
});

Or use _.merge on the entire DEFAULT_OPTIONS: https://github.com/sumul/fractal-react-adapter/blob/master/src/adapter.js#L117