Add hook for img.php before using CImage

[mosbth]

Added a hook where one can create an own function to do calculations and change the settings, before CImage is called.

      /**
       * A function (hook) can be called after img.php has processed all
       * configuration options and before processing the image using CImage.
       * The function receives the $img variabel and an array with the
       * majority of current settings.
       *
       * Default value:
       *  hook_before_CImage:     null
       */
       /*'hook_before_CImage' => function (CImage $img, Array $allConfig) {
           if ($allConfig['newWidth'] > 10) {
               $allConfig['newWidth'] *= 2;
           }
           return $allConfig;
       },*/

This can be used to implement #118 or equal logic that could be debatable if it should be in img.php/CImage or not.