Cluster mode

How to run Vegeta across multiple machines:
* Ship a vegeta binary to all the machines, accessible in PATH
* Use some sort of orchestration tool such as pdsh to run the same attack in parallel on those machines
* Collect the results into a single machine and use vegeta report on them

```shell
$ vegeta report -reporter=plot:timings -input=results1.vr,results2.vr,results3.vr > plot.svg
```