Restore timestamps from older chats.

[jkoornneef]

Can addResponseMessage and addUserMessage be extended to pass in an optional timestamp Date.
I would like to continue a previous chat. I can load the history from the database and use those two methods, but all messages will have the current time stamp due to:
timestamp: new Date(),

[bnjr]

There is a PR for this #275
@mcallegari10 request you to merge as it is a requirement from lot of us

[jkoornneef]

@bnjr Thanks, I missed that branch. I agree, a merge would be helpful for many.