Have Hubot repeat sender names

[codymcowan]

In many chat applications, when using this tool a user will receive the notification from Hubot, and see only the message Hubot sends (which as currently designed does not include who sent the original message). Having Hubot repeat the name of the sender, as shown below, would provide more context to those receiving a Hubot notification.

Current:

Alice> Hey @dev, there's a standup in 5 min.
Hubot> Hey @Bob @Eve, there's a standup in 5 min.

Proposed

Alice> Hey @dev, there's a standup in 5 min.
Hubot> Alice: Hey @Bob @Eve, there's a standup in 5 min.

[cycomachead]

Yes, I think this makes sense. (It was once on one of my TODO lists...)

I don't have much time right now, but I would certainly welcome a PR. :)