code: function addPermission(rbac, action, resource, cb) {
    rbac.getPermission(action, resource, (err, permission) => {
      if (err) {
        return cb(err);
      }
  
      if (!permission) {
        return cb(new Error('Permission not exists'));
      }
  
      if (indexOf(this.permissions, permission.name) !== -1) {
        return cb(new Error('Permission is already assigned'));
      }
  
      this.permissions.push(permission.name);
      return this.save((err2, user) => {
        if (err2) {
          return cb(err2);
        }
  
        if (!user) {
          return cb(new Error('User is undefined'));
        }
  
        return cb(null, true);
      });
    });
  
    return this;
  }
docs: Assign additional permissions to the user
  @param  {String|Array}   permissions  Array of permissions or string representing of permission
  @param  {Function} cb Callback