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`.