Add PNG and JPEG to WebP transcoding

WebP lossy mode offers up to 30% better compression than JPEG.

transcoder/util.go

@@ -0,0 +1,15 @@
+package transcoder
+
+import (
+	"net/http"
+	"strings"
+)
+
+func SupportsWebP(headers http.Header) bool {
+	for _, v := range strings.Split(headers.Get("Accept"), ",") {
+		if strings.SplitN(v, ";", 2)[0] == "image/webp" {
+			return true
+		}
+	}
+	return false
+}