Add cp / mv behaviour

[giftig]

Kind of shocked I didn't do this before.

[giftig]

There's put / get for local-to-s3 and s3-to-local moves, which is probably what confused me when I wrote this issue; having the different keywords makes autocompletion easier. cp / mv might still make sense for copying or moving keys s3-to-s3, but probably has a bit less utility so not sure it's worth adding. I'll revisit this one.