Preview of Man page generation for clap definition

[killercup]

Subset of #38

• define a small but somewhat stable API
• example crate that shows the full workflow ("with a build script and everything")

cf. https://github.com/rust-clique/man

[yoshuawuyts]

Feel free to assign this to me.

[killercup]

Somewhat related as it uses similar code, I wrote a small lib for writing a subset of clap definitions to markdown: https://github.com/rust-clique/clap-md

Maybe we can get a small example project going for Preview 2 that showcases both outputs?

[killercup]

code exists and does useful things, but is not released.

see https://github.com/killercup/clap-man-demo for a demo