Add variadic arguments to commands and options. see #53

Will be easier to handle sub-commands that accepts multiple arguments, like:

    program
      .command("new <NAME> [FILES...]")
      .action(function(name, files) {
        console.log(files);
      })
      .parse(process.argv);

    $ program new cool foo bar bad
    >  ["foo", "bar", "baz"]

Added same functionality for Options as well:

    program
      .option('-f, --files <paths...>', 'file paths')
      .parse(process.argv);
    console.log( program.files );

    $ program -f foo.js bar.js baz.js
    >  ["foo.js", "bar.js", "baz.js"]