Feature Request: Update webpack example

[rowild]

Since extract-text-webpack-plugin is deprecated, it would be great to provide an update-to-date example using the recommended mini-css-extract-plugin.

Also, in the readme there is a link to an example, but it leads to a 404.

And what does // do something to collect the whitelist mean? https://purgecss.com/plugins/webpack.html#options

Thank you!