Move to a better zsh implementation

[rjmholt]

Currently the zsh implementation is unstable because it relies on a hacky terminal capture functionality in zsh.

There may be other, better ways to do this, but implementing them requires some deeper knowledge of zsh.

• SO answer on how to capture completions

I know there are some mailing list responses out there that go into other ways to do this but can't find them right now...