$color: green;

h1 {
  background-color: $color;
}

let vendor = true; // this will be configured outside

if (vendor) {
  let style = require('style-loader/useable!css-loader!./file.scss');
  style.use(); //loads just if I call `use`, otherwise not
}

require('my-other-file.scss'); // loads normally, without `use`.

const ExtractTextPlugin = require('extract-text-webpack-plugin')

module.exports = {
  // test: /.(scss|sass|css)$/i,
  test: /^(?!.*style-loader/useable!).*.(sass|scss|css)$/i,
  use: ExtractTextPlugin.extract({
    fallback: 'style-loader',
    use: ['css-loader', 'postcss-loader', 'sass-loader']
  })
}