Support global/flexible position-independent command-line options

[fadeevab]

It makes argh to support "global arguments" like --verbose/-v options, which are position-independent and subcommand-independent.

Motivation
Most competitors like clap support "flexible" arguments positioned in any place of the command line, while argh feels pretty much limited in this case. In contrast, argh is more limited and typically expects global options to appear before any subcommand, which often feels unnatural and clunky.

Before

cli --verbose jump --height 5 ✅
cli jump --verbose --height 5 ❌
cli jump --height 5 --verbose ❌

After

cli --verbose jump --height 5 ✅
cli jump --verbose --height 5 ✅
cli jump --height 5 --verbose ✅

Algorithm

1. #[argh(global)] on the top-level command makes all its options position-independent.
2. The top-level parent options are passed down to subcommands' parsing recursion.
3. If a switch is not found, it's being searched in the parent's options.

[fadeevab]

@elipsitz Hi, I believe this feature is long overdue. Could you please take a look?

[elipsitz]

@fadeevab I’m not a maintainer of this library, I think you meant to tag someone else.

[fadeevab]

@erickt Hi, I believe this feature is long overdue. Could you please take a look?

P.S.: @elipsitz Yeah, sorry, it was a misclick on the GitHub user suggestion.

[fadeevab]

@erickt I updated the motivation for the PR with the examples:

Before

cli --verbose jump --height 5 ✅
cli jump --verbose --height 5 ❌
cli jump --height 5 --verbose ❌

After

cli --verbose jump --height 5 ✅
cli jump --verbose --height 5 ✅
cli jump --height 5 --verbose ✅

[fadeevab]

@erickt What are your overall thoughts? Should it make sense? Or do you have the least changes policy now?