3.0 release plan

[mastilver]

BREAKING CHANGES:

• [x] stop supporting node@6
• [x] stop support webpack 2 and 3
• [ ] fix #144

Fixes:

• [x] fix #167

Chores:

• [x] use MiniCssExtractPlugin instead of ExtractTextPlugin in tests so it's easier to track and fix issues like #176

[Aghassi]

Cross linking https://github.com/danethurber/webpack-manifest-plugin/issues/202, as it applies to Webpack 5 and v3 support

[piecyk]

@mastilver how can i help to land this ?

[shellscape]

Hey folks, please see #222 for updates on v5 support.