Generate several sealed certs from multiple certificates

**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 https://github.com/bitnami-labs/sealed-secrets/issues/263

Start at the kubeseal CLI entry point and trace how the --cert option is parsed and how JSON output is assembled. Check the existing single-certificate behavior first; done means accepting multiple certificates in one command and emitting a JSON array of sealed secrets while preserving the current single-certificate behavior.

Written by the indexing model from the issue text.