Support for lossless WEBP images

Recently I've been pointed to a great project that also deals with image size. I was curious as to how the support for some of the formats I don't include works, and while looking at that I noticed there was a type of webp I had never encountered before in the wild. In order to be thorough, I've patched up this missing hole which means lossless webp images are now supported.

As for the other missing formats, out of the ones they have that this library doesn't the only reasonable candidate for inclusion would be j2c, jp2, jcx, and mng, but I have personally never seen these formats and I don't want to include something unless it's actually used.

Internally there was also some cleanup regarding the interfaces and organization of tests.