Add --fail-if-*-methods options (Closes #347)

We now have two more flags on the CLI:

```sh
bin/atoum --fail-if-void-methods

bin/atoum --fail-if-skipped-methods
```

Using either of those option will turn the result ribbon red if void
and/or skipped methods are found. atoum will also exit with an error
status (1) just like when it normally fails.