Add silence ping logging flag using ExcludePath

- Add `ping-path` option to enable switching on and passing to `logger.go`
  Default remains unchanged at: `"/ping"`
- Add note in configuration.md about silence flag taking precedence

Potential tests:
- `options.go` sets `logger.SetExcludePath` based on silence flag?
- Changing `PingPath` reflected in router?

oauthproxy.go

@@ -254,7 +254,7 @@ func NewOAuthProxy(opts *Options, validator func(string) bool) *OAuthProxy {
 		Validator:      validator,
 
 		RobotsPath:        "/robots.txt",
-		PingPath:          "/ping",
+		PingPath:          opts.PingPath,
 		SignInPath:        fmt.Sprintf("%s/sign_in", opts.ProxyPrefix),
 		SignOutPath:       fmt.Sprintf("%s/sign_out", opts.ProxyPrefix),
 		OAuthStartPath:    fmt.Sprintf("%s/start", opts.ProxyPrefix),