Add ability to specify arbitrary delay unit

[furiousassault]

Hello. Thank you for your project.

AFAIK, now the delay can only be specified in seconds with a minimum 1. Sometimes it's not quite handy, for example in perf tests to set such big delay.

It would be great to have the possibility to set duration as a string, like "300ms" and then parse that duration via ParseDuration instead of reading int and hardcoding * time.Second.
What do you think?

[jmartin82]

It's a great idea, I will add this feature trying to avoid breaking the backward compatibility (at least for now).