Add support for leading comma

Not everyone likes to use trailing comma. Most of the time users' eyes behavior would focus more on the left-hand side of the monitorsand therefore it is very easy to spot on whether it is leading comma or other rust operators, such as =>, +, ., etc.

    match io::stdin().read_line(&mut buffer_input) {
        Ok(buffer_input_size) 
            => println!(
                "Hello, {}! Your name size is {}."
                // call .trim() because .read_line also read `\n` character at the end of input
                , buffer_input.trim()
                , buffer_input_size
            )
        , Err(error) => println!("[Error] Cannot read input. Reason: {}.", error)
    };

Start with the leading-comma example in this issue and review rustfmt's existing comma-formatting behavior; no source file, test, or entry point is named. Done means rustfmt has a defined and consistently supported leading-comma style matching the requested example.