Cluster mode

In order to surpass single machine limits, a cluster mode should be designed.
I think a master-slave architecture would make sense where a the master sends the `Attack` order to the slaves which are listening in some sort of server. The Attacks should be started as synchronised as possible. The overall rate is divided by the number of configured slaves. Vegeta would have to be started in slave mode in the other machines beforehand.
The slaves, once finished, would push the results back to the master (or the master would poll for them after the duration has elapsed).
```shell
$ vegeta slave -port=9999 # on the other machines
$ vegeta attack -rate=25000 -duration=30s -slaves=10.40.32.2:9999,10.40.32.3:9999,10.40.32.4:9999
```