Added zsh completions into project root directory

[Ddfulton]

Try these out with

git checkout feature/zsh_completion
git pull
cat .relate_completions >> ~/.zshrc
zsh  # so that ~/.zshrc re-runs
relate --<TAB>

Of course, these will only work for zsh.

We can also extend them to flag-specific completions, such as --mode followed by one of {All, ..., etc}. Or we can make a version for bash. It depends on what the users want.