Generate Usage spec file

[HipsterBrown]

Is your feature request related to a problem? Please describe.
I've really enjoyed using this library, and I think the docs and parameters data can be used for some interesting integrations, like generating more shell auto-completions, manpages, and Markdown documentation.

Describe the solution you'd like
Provide an option or "plugin" (similar to the existing auto-completion package) that generates a Usage spec file to be used by the usage CLI for accomplishing the feature requests listed above.

Describe alternatives you've considered
Create individual packages that provide each of these features, including extending support for other shells besides bash in the existing auto-completion package.

Additional context
This is an early spec and may be subject to change, so I understand if it's not ready for adding to this core repo.