Add try_from_env() function

[Timmmm]

It would be nice if the library contained this function:

/// Fallible version of argh::from_env().
pub fn try_from_env<T: argh::TopLevelCommand>() -> std::result::Result<T, argh::EarlyExit> {
    let strings: Vec<String> = std::env::args().collect();
    let cmd = cmd(&strings[0], &strings[0]);
    let strs: Vec<&str> = strings.iter().map(|s| s.as_str()).collect();
    T::from_args(&[cmd], &strs[1..])
}

Use case is a bit niche - it's a command that is run with no stdout or stderr so the only way I can see CLI errors is to write them to a file, and I can't do that with argh::from_env().

Also seems a bit weird that argh::EarlyExit uses Result<(), ()> and String rather than just being Result<(), String>.

[shijunti19]

Support does not want to see error output

    let args: Args = argh::from_env();
    println!("args {:?}", args);