GitLab

* Add DS_Store to gitignore

* Add ionic aliases

* Add ionic plugin readme

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>

Fixes #7801

- `custom/example.zshcache` doesn't exist anymore: it's safe to
  ignore.

- `*.swp` files are ignored for no apparent reason. There's nothing
  in the codebase that generates them. This was introduced by @lepht
  in commit f78218f6 on 24 May 2011.

- Files `locals.zsh` and `projects.zsh` are no longer supported for
  putting custom configuration there.

  This is since commit 8debd097 where files were put in a lib/
  folder, but failed to modify the .gitignore file as well.

  The init script was modified as well, so those files aren't even
  being sourced: they are safe to be deleted or ported into a custom
  script.

Instead of .easter-egg files we now have empty .gitkeep files,
like is done as an informal standard.

(Sorry my bad).