Feature request: Support decrypting multiple files at a time

It would be awesome if sops supported decrypting multiple files at a time.

Use case: For example, say you have multiple encrypted Kubernetes secrets that you'd like to apply all at once, this could be streamlined as:

sops -d secrets/*.enc.yml | kubectl apply -f -

This would be much easier than having to manually specify each file individually, one at a time. This would also require that sops be able to concatenate each file using the --- document delimiter in the resulting YAML stream.

Start at the sops -d command's handling of input files and trace the existing single-file decryption path. Define how multiple inputs should be separated with YAML document delimiters, then verify that the resulting stream can be consumed by kubectl apply -f - for multiple Kubernetes secrets.