code: function doPostCss(css, callback) {
  	postcss([autoprefixer]).process(css, {from: undefined}).then(function gotCssResult(result) {
  		callback(null, result.css);
  	}).catch(function gotError(err) {
  		callback(err);
  	});
  }
docs: Apply postCss
  
  @author   Jelle De Loecker   <jelle@develry.be>
  @since    0.5.0
  @version  1.0.5
  
  @param    {String}     css
  @param    {Function}   callback