Add a App::unset_setting or App::unset_settings fn

[brennie]

Currently if you would like to disable a setting, you have to use undocumented interfaces to do so. I would prefer to not have the --help option and instead only use the help subcommand so I currently have to do:

let mut app = App::new("program")
    /* ... */;

app.p.unset(AppSettings::NeedsLongHelp);

It would be much more ideal to do:

let app = App::new("program")
    .unset_setting(AppSettings::NeedsLongHelp)
    /* ... */;

[kbknapp]

Good idea, thanks! 👍

I'll add this to the queue, and should a very easy addition if anyone would like to try out this as a first PR.

[brennie]

I would be willing to take this on.