Guard zsh files with zsh-completion USE flag
Merge branch 'zsh-completion-use-flag' of https://github.com/uggedal/docker-overlay

* 'zsh-completion-use-flag' of https://github.com/uggedal/docker-overlay:
  Guard zsh files with zsh-completion USE flag
update Manifest and 0.7.4 to include zsh-completion use flag