Add a way to disable all default filters and really provide a whitelist
minor #731 Add a way to disable all default filters and really provide a whitelist (Seldaek)

This PR was squashed before being merged into the 1.1-dev branch (closes #731).

Discussion
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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).

Commits
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669e8f8 Add a way to disable all default filters and really provide a whitelist