...
  module: {
    loaders: [
      {
        test: /\.css$/,
        loader: 'css!clean-css'
      },
      {
        test: /\.styl$/,
        loader: 'css!clean-css!stylus?reslve url'
      }
      ...
    ]
    // Example Set options (Key "clean-css" or cleancss or CleanCSS):
    "clean-css":{
      debug:true,
      mediaMerging: true
    }
  }