Can't modify webpack configuration: .default is not a valid Plugin property

Hi,

My conf is: Win7, node v8.12.0, yarn 1.9.2. Here's what I did :

git clone https://github.com/electron-userland/electron-webpack-quick-start.git my-app

I changed `package.json`, adding:

```json
{

"electronWebpack": {
"renderer": {
"webpackConfig": "webpack.renderer.additions.js"
}
}
}
```
My webpack.renderer.additions.js contains just nothing for the moment (to be sure the problem doesn't come from any third party lib, as you see I plan to use handlebars):

```js
module.exports = {
module: {
rules: [
// {
// test: /\.hbs$/,
// use: ['handlebars-loader']
// }
]
}
}

```

I cleaned the `src/renderer/index.js` => blank

and when I do `yarn dev`, I get :
```shell
ERROR in ./src/renderer/index.js
Module build failed (from ./node_modules/babel-loader/lib/index.js):
Error: [BABEL] D:\...\my-app\src\renderer\index.js: .default is not a valid Plugin property
```

How can I fix the problem?
When I remove "electronWebpack..." from package.json, it works fine. It seems I can't modify the webpack conf...