Accept more types of positional arguments

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:
```txt
SYNOPSIS
          my_cmd [OPTIONS] arg1 arg2? arg3...
```

## TODOS
- [ ]  design API
- [ ]  implement