Add chapter on commandline arguments

Please let me know if you think the example should be more verbose.
Yes. Would you prefer it to only use `println!`?


2014-06-30 20:59 GMT-04:00 Jorge Aparicio <notifications@github.com>:

> In examples/staging/arg/getopts/echo.rs:
>
> > +            os::set_exit_status(1);
> > +            return;
> > +            // The exit code is 0 (success) by default.
> > +            // Any exit code other than 0 indicates failure.
> > +        }
> > +    };
> > +
> > +    if matches.opt_present("help") {
> > +        //^ We could as well have used the short name.
> > +        println!("echo {:s} - display a line of text", VERSION);
> > +        println!("");
> > +        println!("Usage:");
> > +        println!(" {0:s} [SHORT-OPTION]... [STRING]...", program);
> > +        println!(" {0:s} LONG-OPTION", program);
> > +        println!("");
> > +        println(getopts::usage("Echo the STRING(s) to standard output.", opts)
>
> Just spotted the println. I suppose its use is just to avoid the ("{}",
> ... part of the println! call, right?
>
> —
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> <https://github.com/japaric/rust-by-example/pull/76/files#r14384615>.
>