devServer.before and devServer.after method does not exist

[nashaofu]

devServer has no before and after methods. I made a mistake when I was using it. thanks！
https://webpack.js.org/configuration/dev-server/#devserverbefore

[eliperelman]

When a method doesn't exist, you can manually set it via .set:

config.devServer.set('before', () => {});
config.devServer.set('after', () => {});

We would welcome a PR to update those methods.

[eliperelman]

Fixed by #167.