Repeated arguments on the CLI

This PR lets atoum handle repeated arguments on the command line. For example:

```sh
$ 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 on the command 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.

Here is a more detailed example to illustrate the use case of this PR:

```sh
# With atoum' actual behavior
bin/atoum -c .atoum.foo.php -c .atoum.bar.php --test-all

# => Including .atoum.php if it exists
# => Including .atoum.bar.php
```

Actually, only the last ```-c``` is applied (they are both applied but the last one overrides the first). A workaround can be used but it's a bit hacky:

```sh
# With atoum' actual behavior
bin/atoum -c .atoum.foo.php --configurations .atoum.bar.php --test-all

# => Including .atoum.php if it exists
# => Including .atoum.foo.php
# => Including .atoum.bar.php
```

See the hack here ? We use the two forms, short and long, of the argument so that they both get applied.
To cleanly get the expected behavior we could use this:

```sh
# With atoum' actual behavior
bin/atoum -c .atoum.foo.php .atoum.bar.php --test-all

# => Including .atoum.php if it exists
# => Including .atoum.foo.php
# => Including .atoum.bar.php
```

And it works :) But this has a main drawback in the context f Netbeans integration : it would force NB team to implement something that will check if the user (when customising atoum call) has included a configuration file with the ```-c``` or ```--configurations``` options and:
* With the hacky way : use the unused arg form to include the Netbeans configuration file
* Find the ```-c/--configurations``` in the user command line and append a path to its value

I don't think both these solutions are good so I tried to propose something which I think is better...

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 above.

Hope it helps, at least to find a better solution if this one is not good :)