Added v as a keybinding to edit-command-line in vi-mode

taken from [here](http://www.zsh.org/mla/users/2008/msg01013.html)
We want pressing v in command mode to take us to an editor (defined by the $VISUAL or $EDITOR variable or vi) to edit the current contents of the command line which (I believe) is a standard feature of bash vi-mode or other readline prompts (see [here](http://www.catonmat.net/download/bash-vi-editing-mode-cheat-sheet.pdf) and [here](http://www.catonmat.net/blog/bash-vi-editing-mode-cheat-sheet/) for source).