Use stdio to parse options

Big changes:
* Option parsing now done with [stdio](https://github.com/sgmonda/stdio). Adding a `true` after any option is not needed anymore.
* Changed option `--rps` to mean total requests per second, not rps per client as before.
* Option `--agent` is deprecated in favor of `--keepalive`.

Version 1.0.0 ready for immediate merge. Sending a PR in case anyone wants to take a look after the fact.