code: function plugin() {
    return function(style){
      style.include(__dirname);
  
      if (Canvas) {
        style.define('has-canvas', nodes.true);
  
        // gradients
        style.define('create-gradient-image', gradient.create);
        style.define('gradient-data-uri', gradient.dataURL);
        style.define('add-color-stop', gradient.addColorStop);
  
        // color images
        style.define('create-color-image', colorImage.create);
        style.define('color-data-uri', colorImage.dataURL);
      } else {
        style.define('has-canvas', nodes.false);
      }
    };
  }
docs: Return the plugin callback for stylus.
  
  @return {Function}
  @api public