Support multiple data files in helm generate headers

[arschles]

For example: I'd like to have two separate data files to store:

1. docker image tag values and imagePullPolicy values
2. external object storage configuration data

One idea to support this would be to allow helm template to look for multiple -d parameters. That solution could look like the following i a helm:generate header:

//helm:generate helm tpl -o path/to/file.yaml -d docker_values.yaml -d object_storage_values.yaml /path/to/template_file.yaml