Split test pipeline stages

TODO: Adapt README to reflect staged CLI

In order to generate more than one report for the same `Results`, an option to save an intermediate representation should be added.
This leads to the somewhat obvious introduction of sub-commands on the CLI that handle different aspects of a test pipeline.
```shell
$ vegeta attack -rate=50 -duration=10s | vegeta report -reporter=text
$ vegeta attack -rate=50 -duration=5s > results.vr
$ vegeta report -input=results.vr -reporter=text
$ vegeta report -input=results.vr -reporter=json
```