code: function use(plugin, options){
    utils.assertString(plugin, 'plugin');
  
    if (options) {
      utils.assertType(options, 'object', 'options');
      options = parseObject(options);
    }
  
    // lookup
    plugin = plugin.string;
    var found = utils.lookup(plugin, this.options.paths, this.options.filename);
    if (!found) throw new Error('failed to locate plugin file "' + plugin + '"');
  
    // use
    var fn = require(path.resolve(found));
    if ('function' != typeof fn) {
      throw new Error('plugin "' + plugin + '" does not export a function');
    }
    this.renderer.use(fn(options || this.options));
  }
docs: Use the given `plugin`
  
  Examples:
  
  use("plugins/add.js")
  
  width add(10, 100)
  // => width: 110