Quality setting is being ignored for formats other than JPEG #31

A second discovery while learning the API, and evaluating it.

tnail = get_thumbnail(testfilename, '300x300', crop='center', force=True, quality=100)
82K
tnail = get_thumbnail(testfilename, '300x300', crop='center', force=True, quality=10)
5K

tnail = get_thumbnail(testfilename, '300x300', crop='center', force=True, format="PNG", quality=10)
148K

tnail = get_thumbnail(testfilename, '300x300', crop='center', force=True, format="PNG", THUMBNAIL_FORCE_FORMAT="PNG", quality=100)
148K

Pillow does allow the setting of quality for PNG's, which I use in my library. And I see the change in JPG format, so I'm not sure why this wouldn't be working for alternative graphic formats.