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.

plugins/cp/cp.plugin.zsh

@@ -1,14 +1,4 @@
-#Show progress while file is copying
-
-# Rsync options are:
-#  -p - preserve permissions
-#  -o - preserve owner
-#  -g - preserve group
-#  -h - output in human-readable format
-#  --progress - display progress
-#  -b - instead of just overwriting an existing file, save the original
-#  --backup-dir=/tmp/rsync - move backup copies to "/tmp/rsync"
-#  -e /dev/null - only work on local files
-#  -- - everything after this is an argument, even if it looks like an option
-
-alias cpv="rsync -poghb --backup-dir=/tmp/rsync -e /dev/null --progress --"
+cpv() {
+    rsync -pogbr -hhh --backup-dir=/tmp/rsync -e /dev/null --progress "$@"
+}
+compdef _files cpv