Before Styles Function

Function to be performed on the image before resizing it.
Very useful to put watermark on all images before resizes them.

```php
'pre_styles' => function ($file, $imagine) {

			$watermark = $imagine->open(
				base_path('wartermarker/acn.png')
			);
			$image = $imagine->open($file->getRealPath());
			$size = $image->getSize();
			$watermarkSize = $watermark->getSize();
			$bottomRight   = new Imagine\Image\Point($size->getWidth() -  $watermarkSize->getWidth(),    $size->getHeight() - $watermarkSize->getHeight());

			$image->paste($watermark, $bottomRight);

			return $image;
		},
```
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Sorry if my choice is not very good. 
I had the need and the idea and decided to implement. 
Lay the layout of the community as to whether this is a useful or viable option.