A very basic POC. Need to refactor to generate completion
structure before passing to the template to avoid repeated
computations.
What works:
* Real zsh completion (not built on bash)
* Basic flags (with long flag and optional shorthand)
* Basic filename completion indication (not with file extensions though)
What's missing:
* File extensions to filename completions
* Positional args
* Do we require handling only short flags?
