Fix cp plugin completion and refactor (#5427)

* cp plugin: change cpv to function so that completion works

* cp plugin: show numbers in units of 1024 (K,M,G,T)

Use `-h` level (3): output numbers in units of 1024.
See the manpage of rsync for more information.

* cp plugin: add a README file

* cp plugin: recurse directories

* cp plugin: remove `--` to separate files from options

This has some undesired effects, like having `cpv --help` be a file
not found error.

Use `--` yourself if you need it (which you generally don't):

```zsh
cpv -- -some-file-with-hyphens.txt /tmp
```

Added this same info to the README.