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Use add-zsh-hook to add functions to hooks. That way they won't be added again
when doing `source ~/.zshrc` multiple times.
Co-authored-by: Marc Cornellà <marc.cornella@live.com>

See https://github.com/robbyrussell/oh-my-zsh/pull/6533#issuecomment-360878060

 Changes to be committed:
	modified:   dirhistory/dirhistory.plugin.zsh

New Feature:
  Navigate directory hierarchy using ALT-UP and ALT-DOWN. (mac keybindings not yet implemented)
  ALT-UP moves to higher hierarchy (cd ..)
  ALT-DOWN moves into the first directory found in alphabetical order

Add shortcuts for mac keyboards as an alternative to alt+left
abd alt+right: mac users can now use opt+left and opt+right.

This avoids spawning additional processes as $PWD **always** contains
the current working directory.

This plugin navigates directory history using ALT-LEFT and ALT-RIGHT.