add webp support (decode only)

if any transformation is requested, webp images will be encoded and
served as jpeg or png, defaulting to jpeg if no format is specified.

Fixes #88

README.md

@@ -4,7 +4,7 @@ imageproxy is a caching image proxy server written in go.  It features:
 
  - basic image adjustments like resizing, cropping, and rotation
  - access control using host whitelists or request signing (HMAC-SHA256)
- - support for jpeg, png, and gif image formats (including animated gifs)
+ - support for jpeg, png, webp (decode only), and gif image formats (including animated gifs)
  - on-disk caching, respecting the cache headers of the original images
  - easy deployment, since it's pure go
 
@@ -197,6 +197,15 @@ However, you can use the `scaleUp` command-line flag to allow this to happen:
 
     imageproxy -scaleUp true
 
+### WebP support ###
+
+Imageproxy can proxy remote webp images, but they will be served in either jpeg
+or png format (this is because the golang webp library only support decoding)
+if any transformation is requested.  If no format is specified, imageproxy will
+use jpeg by default.  If no transformation is requested (for example, if you
+are just using imageproxy as an SSL proxy) then the original webp image will be
+served as-is without any format conversion.
+
 ## Deploying ##
 
 You can build and deploy imageproxy using any standard go toolchain, but here's