[Feature] s5cmd cp all versions of a single key

[skeggse]

I sometimes have a bucket with versioning enabled, and I want to get copies of all the versions of a given object key. I can do that by using aws s3api list-object-versions, extracting the VersionId, and then execing s5cmd cp --version-id <id> over all the versions, but this seems like a problem s5cmd could solve directly - provide a --all-versions option, or accept some --versions-file containing all the versions I want to download - without requiring a bunch of process execution overhead!