Added ability to set global sanitizers.

I've added a feature that allows a user to set global sanitizer rules to be applied to every item in the dataset. Simply set an item in the rules array that uses `*` as the key, for example:

```php
$rules = [
    '*' => 'trim|strtolower',
    'last_name' => 'strrev',
];
```

Global rules will then be processed prior to the field-specific rules. Does this sound useful to you?