SUGGESTION: Allow prune per compile output

[ademariag]

Describe the bug/feature
Sometimes global prune on/off is not enough.

I propose to add the optional "prune" option to a compile configuration:

    kapitan:
      compile:
        - output_path: terraform
          output_type: json
          output_prune: false
          input_paths:
            - components/terraform/main.jsonnet
          input_type: jsonnet

Additional context
In particular, to give a use case, I compile terraform together with spinnaker and kubernetes, and as for this "bug" hashicorp/terraform-provider-google#4431 terraform sometimes needs to see the nulls