Automatic merge from submit-queue

add kubectl cp

Implements `kubectl cp` (https://github.com/kubernetes/kubernetes/issues/13776)

Syntax examples:

```sh
# Copy from pod to local machine
$ kubectl cp [namespace/]pod:/some/file/or/dir ./some/local/file/or/dir

# Copy from local machine to pod
$ kubectl cp /some/local/file/or/dir [namespace/]pod:/some/remote/file/or/dir
```

@deads2k @smarterclayton @kubernetes/sig-cli