Added 'unwrap_or_exit'.

[goyox86]

In the coreutils and userutils and pretty much all the crates that
implement CLI applications is very common to execute system calls or
other library code that returns optional types and inmediately exit the
program in case of an None or an Err is found displaying a message
to STDERR with something like "foo: file not found" where 'foo' is the
utility.

In code it looks like this:

let user = get_user_by_id(uid).unwrap_or_else(|err| {
  eprintln!("passwd: {}", err);
  exit(1);
})

And after this commit we could do:

let user = get_user_by_id(uid).unwrap_or_exit(1);

@MggMuggins came with a proposal in #10
(in which this work is based on) and after discussing we agreed to
implement unwrap_or_exit.

[jackpot51]

Thanks!