init command should create config with mini-css-extract-plugin

[jevancc]

Do you want to request a feature or report a bug?
Feature

What is the current behavior?
The init command creates config with extract-text-webpack-plugin to extract CSS into separate files when used in production. However, it should use mini-css-extract-plugin since ETWP no longer supports webpack@4.

What is the expected behavior?
Use mini-css-extract-plugin instead of extract-text-webpack-plugin in generated config file.

If this is a feature request, what is motivation or use case for changing the behavior?
Supporting the new MiniCssExtractPlugin.

[evenstensberg]

#373