Bring flags in line with crontab command

[jacobtomlinson]

One of the goals of krontab is to replace crontab for systems which live on a Kubernetes cluster (e.g pangeo/z2jh). Therefore ideally I would like it to have a superset of crontab's flags.

Looking at the crontab man page there are some flags which haven't been implemented yet.

Flags

• -l List the crontab. This should list all kubernetes CronJob resources in a crontab format.
• -e Edit the crontab. This should open all kubernetes CronJob resources in an editor (either system default or that configured with VISUAL/EDITOR) in a crontab format. Then changes should be propagated back to the Kubernetes API.
• -u Modify the crontab of another user. This could be used to override the KRONTAB_OWNER config setting.
• -r Remove the current crontab. This should delete all the CronJob resources, however this is a bit scary ⚠️. We should probably prompt the user to confirm.
• -i Modifies the -r flag to prompt the user to confirm y/n. This should possibly be the default and use a -y to force without confirmation. Therefore maybe we should just stub this out.
• -s Changes the SELinux context of the crontab file before editing. Probably not necessary in krontab but we should stub it out.