Per directory history plugin

   This is a implementation of per directory history for zsh, some
   implementations of which exist in bash[1,2].  It also implements
   a per-directory-history-toggle-history function to change from using the
   directory history to using the global history.  In both cases the history is
   always saved to both the global history and the directory history, so the
   toggle state will not effect the saved histories.  Being able to switch
   between global and directory histories on the fly is a novel feature as far
   as I am aware.

   [1]: http://www.compbiome.com/2010/07/bash-per-directory-bash-history.html
   [2]: http://dieter.plaetinck.be/per_directory_bash