Add an option to append .webp instead of changing it

It makes the file much 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