feature request: grouped flag categories in help output

[tylerbutler]

It can be helpful in some situations to group flags together into groups in the help output. For example, a CLI might have a set of global flags like --quiet and --verbose that apply to all commands. As flags are layered on from subcommands, it's easy to get lost in the sea of flags in the output.

Imagine something like this:

FLAGS:
    --allow=<STRING_LIST>   Allow licences, comma-separated
    --config=<STRING>       Read configuration from PATH
    --deny=<STRING_LIST>    Deny licences, comma-separated
    --ignore-config=<BOOL>  Ignore configuration files
    --manifest=<STRING>     Read manifest from PATH

CACHE FLAGS:
    --cache-path=<STRING>   Override the licence metadata cache file location
    --no-cache=<BOOL>       Bypass the on-disk licence metadata cache

GLOBAL FLAGS:
    --color=<STRING>        Colorize output: auto|always|never (default auto)
    --help                  Print help information
    --quiet=<BOOL>          Suppress progress output
    --verbose=<BOOL>        Show detailed progress output

My suggestion is to add the concept of a group that potentially only applies to the help output. I'm not sure how it would work exactly, but if you're open to the feature I could explore in a PR.