Add Arg::conflicts_with_everything method

[CreepySkeleton]

Summary

Add explicit support for arguments that conflict with every other flag and must be always passed alone.

Detailed description and motivation

There are cases when we need to introduce a command line argument that should always be used standalone, i.e it conflicts with every other argument.

For now this can be implemented via conflicts_with_all but we have to explicitly enlist all the other flags then:

App::new("My daemon")
    .version("1.0")
        // this flag should be always passed alone, so it conflicts with any other flag
        .arg(Arg::with_name("status")
        .conflicts_with_all(&[
            // for now there's only one another flag so this is fairly simple
            // but once we have other arguments we have to pay some extra attention 
            // to not miss some accidentally          
            "SERIAL-PORT",
        ])
        .short("s")
        .long("status")
        .help("Daemon status"))
    .arg(Arg::with_name("SERIAL-PORT")
        .help("Sets the serial-port to use")
        .required(true))
    .get_matches();

Inspired by #1579