add support for a local-overwrite in .php_cs

This allows to do local overwrites, for the IDE or experimental folder.
Or when using a local library-folder for none composer manageable stuff.

local.php_cs can use the `$finder` to add additional excluding.

```php
<?php

$finder
    ->exclude('.idea')
;
```
Merge pull request #10 from sstok/patch-1

add support for a local-overwrite in .php_cs