Reduces bundle size for modern browsers from about 483,477 bytes
(uncompressed) to 392,782 bytes (uncompressed). After gzip, it's 121,228
bytes down to 100,848 bytes.

Discovered that ExtractTextPlugin is deprecated for recent Webpack, so
switched to MiniCssExtractPlugin. Also, switched to contenthash for
better caching and because otherwise I end up writing the same extracted
CSS twice under two different hashes (I'm still writing it twice, but at
least it's idempotent now).

Closes: https://github.com/wincent/masochist/issues/133