Add quality key to description

This PR adds the ability for a client to set the quality of individual thumbnails to a higher or lower value, allowing an individual thumbnail to have a smaller file size at the cost of image quality, making the image a faster download. This feature can be important for image heavy pages.

The quality of the thumbnails being generated are [automatically determined](http://www.imagemagick.org/script/command-line-options.php#quality) by default. 

In my tests, resizing a large JPG photo at a quality of `90` can result in a file size reduction of almost 2x over the default quality setting, making it download almost 2x faster.