Support for unique responses

Feel free to redit or reject this. It works like this:

    + {unique}tell me something interesting
    - I will only say this once!
    - And I will only say this once!

    You> tell me something interesting
    Bot> And I will only say this once!
    You> tell me something interesting
    Bot> ERR: No Reply Found
    You> tell me something interesting
    Bot> I will only say this once!
    You> tell me something interesting
    Bot> ERR: No Reply Found
    You> ^C

Very useful in my situation where I cannot allow certain replies to be produced twice.

I also made a thread here:

http://www.rivescript.com/yabb2/YaBB.pl?num=1364228508