Support proxying images

This should make it faster because the actual favicons are now cached,
too.  Especially over HTTP/2 connections, this should improve the
response times quite a bit.

go/favicon/favicon.go

	"errors"
	"flag"
	"fmt"
	"io/ioutil"
	"net/http"
	"net/url"
	"os"
var faviconCache *lru.Cache
var lock sync.RWMutex
var imageCache *lru.Cache
var imageLock sync.RWMutex
func main() {
	flag.Parse()
	faviconCache = lru.New(*cacheSize)
	imageCache = lru.New(*cacheSize)
	http.HandleFunc("/favicon", HandleGetFavicon)
	http.HandleFunc("/favicon_proxy", HandleProxy)
	if p := os.Getenv("PORT"); p != "" {
		flag.Set("p", p)
	}
	}
}
func HandleProxy(w http.ResponseWriter, r *http.Request) {
	url := r.URL.Query()["url"][0]
	favicon, err := GetFaviconCached(url)
	if err != nil {
		fmt.Printf("Error: '%s': %s\n", url, err)
		w.WriteHeader(http.StatusNotFound)
		w.Write([]byte(fmt.Sprint(err)))
		return
	}
	image, err := GetImageCached(favicon)
	if err != nil {
		fmt.Printf("Error: '%s': %s\n", url, err)
		w.WriteHeader(http.StatusNotFound)
		w.Write([]byte(fmt.Sprint(err)))
		return
	}
	w.Write(image)
}
func GetImageCached(u string) ([]byte, error) {
	imageLock.RLock()
	image, cached := imageCache.Get(u)
	imageLock.RUnlock()
	if cached {
		return image.([]byte), nil
	}
	resp, err := http.Get(u)
	if err != nil {
		return nil, err
	}
	defer resp.Body.Close()
	imageData, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, err
	}
	imageLock.Lock()
	imageCache.Add(u, imageData)
	imageLock.Unlock()
	return imageData, nil
}
func HandleGetFavicon(w http.ResponseWriter, r *http.Request) {
	url := r.URL.Query()["url"][0]
	favicon, err := GetFaviconCached(url)