code: function registerTarget(userId, channel, target, callback) {
      this.save(userId, channel, target, function (err) {
          // ensure that we're firing the callback asynchronously
          process.nextTick(callback, err)
      })
  
      // make it chainable
      return this
  }
docs: Register a new target of a channel.
  
  @param {*} userId - User identifier. Type is mostly string or number but depends on the consumer.
  @param {string} channel - Channel name.
  @param {*} target - An address to send notifications to. Eg. phone number, email address, push notification token, etc.
  @param {function} callback
  @returns {Handover}