Use Clean-CSS instead of cssnano

[matzeeable]

Use https://github.com/leodido/postcss-clean
npm install --save-dev postcss-clean

postcss.config.js

module.exports = ({ file, options, env }) => ({
    plugins: [
        require('autoprefixer')
    ].concat(env === 'production' || env === 'umd' ? [require('postcss-clean')(options.clean)] : [])
});

webpack.config.js

loader: 'postcss-loader',
options: {
	config: {
		ctx: {
			clean: {}
		}
	}
}