Update class.IMagickPreviewer.php

IM_CUSTOM_OPTIONS is used even for thumbs generation, no resize for thumbs (have to be defined in quality parameter).
Parameters values examples :
- IM_CUSTOM_OPTIONS : -strip -alpha opaque
- IM_THUMB_QUALITY : 65 -density 150 -resize 250
- IM_VIEWER_QUALITY : 90 -density 300 -resize 25%