Add option for recurring silences

[jkroepke]

Hi, thanks for your work! I appreciate it.

I have the use case where customer VM starts to scan the whole filesystem at a given time each week. Currently, we have a Kubernetes based shell script which does this for us. However it would be nice to have a other kind like RecurringSilence which creates a Silence based on a cronjob expression and spec.template.

For cron, I may recommend https://github.com/aptible/supercronic/tree/master/cronexpr. It extends the cron expression to fulfill requirements like "the nearest business day to the 15th of the month."