C-x C-e to edit current command in EDITOR

The command C-x C-e opens the current command line for editing
in the editor defined by the $EDITOR environment variable.

Thanks to Craig Bosma and his blog post about this:
http://distrustsimplicity.net/articles/zsh-command-editing

lib/edit-command-line.zsh

@@ -0,0 +1,3 @@
+autoload -U edit-command-line
+zle -N edit-command-line
+bindkey '\C-x\C-e' edit-command-line