Cover images prioritization

**original issue: #836** 

the plugin now prioritizes the image names that contain substring(s) that comes first in its *cover_names* option list.

Say the plugin config is 

    'cover_names': ['cover', 'front', 'art', 'album', 'folder'],

Rather than running the regex for pattern matching on the raw images listing, we first sort the filenames based on the existence of keywords in the names, eg :  

    upfront_cocover.jpg > cover_art.jpg > front_art.jpg > album_front.jpg etc.

Then the regex is kept to exclude wrong candidates, so in this example cover_art.jpg is the image that would be returned.