Added dirhistory plugin.

This plugin navigates directory history using ALT-LEFT and ALT-RIGHT.
List pkgs by size
Merge pull request #2597 from githubhjs/master

List pkgs by size
Merge pull request #2576 from jeffwilliams/master

Added dirhistory plugin.
Merge branch 'master' of github.com:robbyrussell/oh-my-zsh