Give priority to certain extensions
Give priority to certain extensions

While using PIL's EXTENSION list for our format-to-extension is cool,
simply iterating to get an appropriate extension isn't always desirable
since one of the more obscure extensions may occur first. This commit
allows us to give precedence to certain extensions, but still honors
PIL's EXTENSION dict.

Related: jdriscoll/django-imagekit#186

fladi/pilkit,dulaccc/pilkit,dulaccc/pilkit,kezabelle/pilkit