Add WebP Support

Thanks for the patch komejo. I'm not against this idea, but I think there's a fundamental problem with this implementation.

1. There's an efficiency issue for sites without the webp module. You should check that the webp module is enabled before running the extension checks.
2. If webp images are available, you're not leaving any fallbacks incompatible browsers. You might check out either using a @supports block, or maybe even using an image-set as a source if all browsers that support image-set also support webp: https://developer.mozilla.org/en-US/docs/Web/CSS/image/image-set

Ah, excellent feedback - I'll see if I can find the time to make a better patch!