Make kubeval's directory recursion strategy similar to that of kubectl.

## The suggestion
When pointed to a directory in order to validate its contents, could we please have `kubeval`:

* Process only the immediate children of the specified directory by default.
* Recursively process all children of the specified directory only when passed a parameter like `--recursive`.

This would also bring `kubeval`'s behaviour in concordance with that of `kubectl`.

## The rationale
I have a top level directory containing all the manifests that I'd like `kubectl` to apply. Within this directory is a subdirectory containing ([Sops](https://github.com/mozilla/sops)-)encrypted descriptions of `ConfigMaps` containing sensitive data¹. I'd like to be able to point `kubeval` to the top-level directory and have it validate its immediate children only rather than recurse down to the directory containing the descriptions of the `ConfigMap`s and report a failure.

Thank you.

¹ In my process I'm applying the descriptions of the `ConfigMaps` as a different step, which also decrypts them on the fly.