Add copybuffer function and keybinding (#5431)

* Add copybuffer function and keybinding:

binds ctrl-o to copy the command line buffer to the system clipboard.

* Add README

plugins/copybuffer/README.md

+# `copybuffer` plugin
+
+This plugin binds the ctrl-o keyboard shortcut to a command that copies the text
+that is currently typed in the command line ($BUFFER) to the system clipboard.
+
+This is useful if you type a command - and before you hit enter to execute it - want
+to copy it maybe so you can paste it into a script, gist or whatnot.
+
+```zsh
+plugins=(... copybuffer)
+```

plugins/copybuffer/copybuffer.plugin.zsh

+# copy the active line from the command line buffer 
+# onto the system clipboard (requires clipcopy plugin)
+
+copybuffer () {
+  if which clipcopy &>/dev/null; then
+    echo $BUFFER | clipcopy
+  else
+    echo "clipcopy function not found. Please make sure you have Oh My Zsh installed correctly."
+  fi
+}
+
+zle -N copybuffer
+
+bindkey "^O" copybuffer