Add AppSettings::DisableHyphenSubcommand to allow removing hyphen in help messages

[fanzeyi]

Related to #1431 but this does not completely close the issue as this only implemented the setting for v2.

$ cargo test --test app_settings
...
test result: ok. 63 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.02s

Example:

let res = App::new("myprog")
    .subcommand(SubCommand::with_name("test").setting(AppSettings::DisableHyphenSubcommand))
    .get_matches_from_safe(vec![
        "myprog", "test", "--help"
    ]);
assert!(res.is_err());
assert!(res.unwrap_err().message.starts_with("myprog test"));

[epage]

@pksunkara Whats the bar for features going into v2?

[pksunkara]

We don't allow it. Only security fixes. Please redo the pr for master branch