feat: add webpack-dev-server config entry (#1016)

[dilidili]

Close #1016

[dilidili]

.umirc.js:

export default {
  devServer: {
    watchOptions: {
      poll: true
    }
  }
}

[sorrycc]

这个配置放 af-webpack 里处理就好了。

相关文件：

• https://github.com/umijs/umi/tree/master/packages/af-webpack/src/getUserConfig/configs
• https://github.com/umijs/umi/blob/master/packages/af-webpack/src/dev.js

[dilidili]

@sorrycc added, thanks!