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).
`none` level should not be passed using CLI.

`php-cs-fixer fix --level=psr2` will run all psr2 fixers
`php-cs-fixer fix --level=psr2 --fixers=foo,-bar` will run all psr2 fixers and foo fixer, but no bar fixer
`php-cs-fixer fix --fixers=foo` will run only foo fixers.

no need for level=none in CLI