title: Minify css from webpack's ExtractTextPlugin and style-loader
body: <p>In this tracer repo: <a href="https://github.com/pconerly/libsass-spritesmith-webpack-tracer" rel="noreferrer">https://github.com/pconerly/libsass-spritesmith-webpack-tracer</a></p>
  
  <p>And this line:
  <a href="https://github.com/pconerly/libsass-spritesmith-webpack-tracer/blob/master/webpack.config.js#L82" rel="noreferrer">https://github.com/pconerly/libsass-spritesmith-webpack-tracer/blob/master/webpack.config.js#L82</a></p>
  
  <p>I'm loading <code>.scss</code>, and extracting them into plaintext. I'd also like to minify them--- how do I do that? <code>style-loader</code> doesn't seem to have an option for it.  Should I be using another plugin like <code>css-loader</code> instead?</p>