Allow overriding parameters in transformation presets

This PR enables overriding of parameters not specified in the transformation preset configuration.

With a configuration like this:

```php
<?php
return array(
    // ...

    'transformationPresets' => array(
        'customThumb' => array(
            'width' => 50,
        ),
    ),

    // ...
);
```

users can adjust the height of the thumbnail through query parameters, but not the width. Before this PR, if a preset had any parameters, the query params were completely ignored.