Add confirmation message to /tell responder

Acknowledged, this breaks the Rule of Silence.  Our team considered doing so a lesser-evil to messages never being delivered because the sender could not know that Hubot had not seen the command. 

I'll let the keepers of the repo determine if they agree enough to merge!
Merge pull request #1227 from BFGeorge9000/patch-1

Add confirmation message to tell responder