don't use style-loader in production

[bj00rn]

style-loader forces inline <style /> tags upon users of this library.

This is a security problem since it prevents the application of sane CSP configuration. ( requires style-src 'unsafe-inline';)

Webpack 4 has the superawesome MinCssExtractPlugin this purpose, this enables users of this library to use the webpack plugin pipline (html-webpack-plugin et al.) to decide if the wan't to inline the library styles or use them in a bundle.

The PR includes

• use MinCssExtractPlugin plugin in production builds - upgrade to Webpack 4 in order to use MiniCssExtractPlugin

[bj00rn]

@fritz-c any thoughts?