feat: true zero-downtime deployment with request draining

[wowu]

Changes:

• Add --pre-stop-hook argument.
• Describe in README how to setup a true zero-downtime deployment.

Notes:

• Hook name "pre-stop" (instead of "before-stop") is inspired by watchtower hooks. This way other hooks can be named "post-stop" etc
• I decided not to add other hooks yet - we will add them when somebody reports a need with a valid use case
• I think it's better to implement draining with an explicitly written command in a hook, rather than a flag like --drain with the same implementation, so users can actually understand the process

Close #21, #32.

[akoenig]

Thanks, @wowu. This looks fantastic! I'm looking forward to the rollout of this feature. 🚀