Add support to configure loglevel

[William Van Hevelingen]

kubexit currently has multiple log.printf statements for informational logs arounds tombstones, birth deps, graveyards and death deps. Currently, these logs are indistinguishable from the logs of the command being run by kubexit which can cause a great deal of confusion and concern for users who see these logs mixed in with the logs of the command they wished to run.

Adding the ability to configure loglevel or adding a prefix so that the logs can be filtered out would be very helpful for operators.

Example:
https://github.com/karlkfi/kubexit/blob/master/cmd/kubexit/main.go#L56