code: function initHandlers(options, handlerClass) {
      var optHandlers = Array.isArray(options[handlerClass]) ?
            options[handlerClass] : [options[handlerClass]],
        handlers = [];
      optHandlers.forEach(function(handler) {
        if (typeof handler === 'function') {
          handlers.push(handler);
        } else if (typeof handler === 'string' && builtinHandler[handlerClass][handler]) {
          handlers.push(builtinHandler[handlerClass][handler]);
        }
      });
      return handlers;
    }
docs: filter() & set builtinHandler