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).
It would be better to said not how to lose sth, but how to gain sth.

It will be great to tell that using a config file the default level is `SYMFONY_LEVEL` and if one would like to use only fixers provided in `->fixers` method one should set alse `NONE_LEVEL`