Add an option to append .webp instead of changing it

It makes the file a lot easier to serve webp using Accept negotiation when the extension is added to the full filename instead of changing it.

http://www.igvita.com/2013/05/01/deploying-webp-via-accept-content-negotiation/
https://github.com/igrigorik/webp-detect/blob/master/nginx.conf

Thank you