Support setting arbitrary metadata

[skeggse]

I've got a use-case where I'm uploading a single file to S3, and I'd like to set additional custom metadata on the object. In this case, it's things like the the checksum of the object's content and the revision in git that produced the artifact.

Would it be acceptable to have additional flags on s5cmd cp to assign arbitrary metadata during object upload?

References:

• How to add custom metadata e.g "cache-control"? #264