Add a way to disable all default filters and really provide a whitelist

I had a big WTF moment because the current readme says this is a whitelist:

```
return Symfony\CS\Config\Config::create()
    ->fixers(array('indentation', 'elseif'))
    ->finder($finder)
;
```

Except it's not, it will first add all `symfony` level fixers and then add those two. So I added a new level to allow choosing exactly what you want from a config file (previously it was only possible with cli args I think).
Please add a test for new level: `FixerTest::getFixerLevels`