Add "fail secure" mode and client timeout

[timonorawski]

• Adds a -failsecure command line parameter that ensures that the target content is always verified to in fact be an image before returning it to the requester - existing functionality acts as an open http proxy with potential for abuse. Note: added this as a command line option so as not to break existing use cases that may depend on the open proxy logic (i.e. using imageproxy as a backend cache for non-image files).
  examples:

• http://localhost:8080/600x/https://www.google.com

• http://localhost:8080/https://www.cnn.com

• applies the Timeout parameter to the http client as well as the server, so as not to leave open connections potentially consuming system resources after we have given up requesting from a slow upstream server.
  e.g. run a nonresponsive http server and then try to imageproxy it

• http://localhost:8080/200x/http://localhost:10000/test.jpg

unresponsivevserver.go:
package main
import (
"net/http"
"time"
"log"
)
func main() {
svr := &http.Server{
Addr:":10000",
Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
time.Sleep(time.Hour)
}),
}
log.Fatal(svr.ListenAndServe())
}

[willnorris]

I'm fairly certain this is addressed by the content-type checking added in 39a4e18 and 0370572. Passing -contentTypes "image/*" should prevent non-image resources from being served.

If there's still more to be done here, please continue the discussion, or re-open the issue.