conversation(recipientId, factory) {
    if (!recipientId || !factory || typeof factory !== 'function') {
      return console.error(`You need to specify a recipient and a callback to start a conversation`);
    }
    const convo = new Conversation(this, recipientId);
    this._conversations.push(convo);
    convo.on('end', (endedConvo) => {
      const removeIndex = this._conversations.indexOf(endedConvo);
      this._conversations.splice(removeIndex, 1);
    });
    factory.apply(this, [convo]);
    return convo;
  }
Starts a new conversation with the user.
@param {Recipient|String} recipientId
@param {Function} factory Executed immediately receiving the convo instance as it's only param.