How to add multi renderer entries support?

I have tried to add `electron-webpack.yml`

```yml
title: true
renderer:
dll: ['vue']
webpackConfig: 'custom.webpack.renderer.js'
```

and the `custom.webpack.renderer.js` content like this

```js
module.exports = {
entry: function () {
return {
entry1: 'src/renderer/entry1.js',
entry2: 'src/renderer/entry2.js'
}
}
}
```
, but compile failed

```
ERROR in multi (webpack)-dev-server/client?http://localhost:9080 webpack/hot/dev-server entry.js
Module not found: Error: Can't resolve 'entry1.js' in 'xxx\electron-webpack-quick-start'
@ multi (webpack)-dev-server/client?http://localhost:9080 webpack/hot/dev-server entry1.js
```