Accept more types of positional arguments

[yoshuawuyts]

https://github.com/rust-clique/man/blob/77e75bb820bff51de3ff2c3472282740b7654579/src/man/mod.rs#L110

Allow accepting not only required positional arguments, also allow optional, and vectors.

The output should be something like:

SYNOPSIS
          my_cmd [OPTIONS] arg1 arg2? arg3...

TODOS

• [ ] design API
• [ ] implement