added support for uri substitution with a random UUID and CID (client ID)

[koAlech]

Hi Uri, what's up? (It's Alik from Ellipsis ... long time!)
This is one cool tool you created!!

I used it to load test our new a/b test API server based on Sixpack.
https://github.com/baloota/sixpack-complete

I added an option on gobench to do 2 uri substitutions:

1. Replace <UUID> with a random UUID
2. Replace <CID> with the client id

I think it is very useful for creating a set of API calls and simulating a complete scenario from each client.
Feel free to comment if you think it could be done better :)

Cheers!