Add silence ping logging flag

Add ability to silence logging of requests to /ping endpoint, reducing
log clutter

Pros:
- Don't have to change all handlers to set/not set silent ping logging
- Don't have to duplicate `loggingHandler` (this could be preferable yet)

Cons:
- Leaking oauth2proxy logic into `package logger`
- Defining default pingPath in two locations

Alternative:
- Add generic exclude path to `logger.go` and pass in `/ping`.

logging_handler.go

@@ -75,18 +75,19 @@ func (l *responseLogger) Status() int {
 	return l.status
 }
 
-// Size returns teh response size
+// Size returns the response size
 func (l *responseLogger) Size() int {
 	return l.size
 }
 
+// Flush sends any buffered data to the client
 func (l *responseLogger) Flush() {
 	if flusher, ok := l.w.(http.Flusher); ok {
 		flusher.Flush()
 	}
 }
 
-// loggingHandler is the http.Handler implementation for LoggingHandlerTo and its friends
+// loggingHandler is the http.Handler implementation for LoggingHandler
 type loggingHandler struct {
 	handler http.Handler
 }