Add a minimum run duration rule

[brianberzins]

A minimum duration rule could be useful to prevent the pod-reaper from killing any pods that are just starting or have only be alive for a short duration.

Example use case: rolling deployments. Having a pod killed during a rolling deployment isn't necessarily bad, but it could cause undesirable effects in the case of automated canary analysis: where a pod being killed could prevent a move forward towards production for no fault of the pod.

[brianberzins]

closing due to temporary insanity, as this actually defines the maximum duration.