Adds ability to configure frequency for pod termination via annotation

[multimac]

Adds ability to configure frequency for pod termination via annotation.

Closes #20

Allows specifying a new command line flag, --termination-frequency-annotation which sets the annotation to look for and use to calculate the approximate frequency at which to terminate a given pod. Follows the examples given in #20, such as...

• chaos.alpha.kubernetes.io/frequency=2/day for "kill this twice per day"
• chaos.alpha.kubernetes.io/frequency=10/hour for "kill this ten times per hour"
• chaos.alpha.kubernetes.io/frequency=1/week for "kill this once a week"
• chaos.alpha.kubernetes.io/frequency=0.5/day for "kill this pod once every two days"

Pods without the annotation are assumed to always be candidates for termination. The --max-kill flag also still applies and will potentially limit how frequently a pod is terminated if it isn't high enough.

[multimac]

Just updated with the ability to set a default termination frequency, via the --default-termination-frequency flag.

This applies to pods which lack the annotation and helps avoid them from being terminated too frequently, since it's likely that people will decrease the interval at which Chaoskube runs when using the annotation.

[multimac]

This PR has ballooned slightly. I've added the ability to configure per-pod (via annotations):

• minimum-age - Overrides the default minimum age
• excluded-weekdays, excluded-times-of-day, and excluded-days-of-year - Global configuration still applies, but can exclude pods from being terminated on additional weekdays, etc.

All of these use annotations in the form chaos.alpha.kubernetes.io/[...]. I've also made a slight change, instead of termination-frequency-annotation the "annotation prefix" can be provided via --config-annotation-prefix, as multiple annotations can now be applied to configure different settings.