code: function normalizeFns(fns) {
    Object.keys(fns).forEach(function(key) {
      var handler = fns[key];
      if (typeof handler === 'function') {
        fns[key] = {
          enter: handler,
          leave: noop
        };
      }
    });
  }
docs: visit can be an object with enter leave or a function if it is a function, run this on enter