Generate several sealed certs from multiple certificates

[ebuildy]

Which component:
kubeseal (CLI)

Is your feature request related to a problem? Please describe.
We run 2 clusters dev , prod with same applications but different kubeseal keys. I want to generate kubeseal secrets with same data for all clusters, in a single CLI command (as a good lazy developer...).

Describe the solution you'd like

CLI option --cert should accept an array.

CLI output should be a JSON array containing the sealed secrets as usually.

Describe alternatives you've considered
Create a small shell wrapper

Additional context

Relates to #263

[guhilling]

I was actually thinking about a similar approach. But then OTOH: The "correct" solution would probably be a plugin for kustomize as described here: #167