Add chapter on commandline arguments

Please let me know if you think the example should be more verbose.
Well, the codes have diverged pretty much: Compare my [echo.rs](https://github.com/vks/rust-by-example/blob/args/examples/staging/arg/getopts/echo.rs) to [theirs](https://github.com/uutils/coreutils/blob/master/src/echo/echo.rs).
The only thing that is really the same is the help message, so maybe we can get away with just using the Rust by Example license?