Allow for command line argument groups

[nderjung]

This issue tracks the ability to add command line argument groups. This is useful to help displaly information on any invocation of the --help flag and to show flags which are part of either a common area, subsystem or category.

For example, all configuration options for KraftKit should be grouped together (see #35) as "global options".

kraftkit/internal/cmdutil/help.go

Lines 53 to 59 in 1b07bef

	// TODO: Introduce flag groupings (e.g. "Global Flags" and "Subcommand Flags")
	flagUsages := command.LocalFlags().FlagUsagesWrapped(80)
	if flagUsages != "" {
	command.Println("\n\nFlags:")
	command.Print(text.Indent(dedent(flagUsages), " "))
	}

[gabrielmocanu]

For this one, we should have something similar with this?

[nderjung]

The output should look similar to this, yes. :D

You can use pflag.SetAnnotation against a flag to attribute it to a group:

https://github.com/spf13/pflag/blob/85dd5c8bc61cfa382fecd072378089d4e856579d/flag.go#L497-L511

to annotate a flag against a group alias name. E.g. the annotation group:global would be the annotation and the "key" would be "global". When the help command is invoked, the key could be looked up against an index of group names, where "global" would correspond to the string "Global flags".

Not sure where group names should be saved though. Perhaps it should be possible via the cmdutil library to register a new group, e.g. a publicaly exposed API cmdutil.RegisterNewFlagGroup(key, title string) which saves to a local variable map which is then referenced (in the same vein as cmdutil.RegisterFlag).