Infer long arguments

[tertsdiepraam]

Closes #1786.

This PR add support abbreviated long options, such as writing --test as --t, --te or --test if the argument is ambiguous. It is enabled with AppSettings::InferLongArgs, modeled after AppSettings::InferSubcommands.

Also, thank you for this amazing project!

[pksunkara]

Does the infer long args work for long flag subcommands?

[tertsdiepraam]

I think that's already covered by InferSubcommands? This PR is compatible with that as far as I can tell. This test from flag_subcommands.rs already existed and still passes with this PR:

#[test]
fn flag_subcommand_long_infer_pass() {
    let m = App::new("prog")
        .setting(AppSettings::InferSubcommands)
        .subcommand(App::new("test").long_flag("test"))
        .get_matches_from(vec!["prog", "--te"]);
    assert_eq!(m.subcommand_name(), Some("test"));
}