Handle repeated arguments on the command line

This PR lets atoum handle repeated arguments on the command line. For example:

```
$ bin/atoum -c atoum.foo.php -c .atoum.bar.php --test-all
# atoum will include .atoum.php (if it exists) and the two other files
```

This PR comes from a request from the Netbeans team (cc @tmysik) which asked me how we could add the coverage report through NB integration of atoum. With this patch, I had a simple idea : atoum would blindly (not really blindly as it keeps everything fine by still cheking args usage) allow the user to repeat arguments so that in Netbeans we would only have to implement a call to atoum and inject a configuration on the fly enabling the coverage report. The user will still be able to customize the call, and inject his own configuration files as atoum will know how to flawlessly handle them all.

I can imagine some other use cases (perhaps not really relevant, I don't know), but this feature is quite restricted to the one I described below.

Hope it helps, at least to find a better solution if this one is not good :)