feat: implement interval jitter

[rarescosma]

Adds a new argument called interval_jitter, expressed as a percentage number (range 0-100) that has the following effects:

If 0 => no effect.

If >0 => choose a random integer in the [-(interval * jitter / 100), +(interval * jitter / 100)] range and use it as an offset for the sleep interval.

So for example, if our interval is 1000ms and we pick interval_jitter=20, then the sleep values will be randomized in the range [800ms, 1200ms]

This has been a requested feature in the original implementation, and is useful to twarth SSH scanners that have tarpit-detecting logic.

Link: https://github.com/skeeto/endlessh/issues/71