oauthutil: tidy interface to Config to add Options struct

The interface was getting so that a new function was needed for every
Config variant. Adding an Options struct fixes this.

backend/pcloud/pcloud.go

@@ -67,7 +67,7 @@ func init() {
 		Description: "Pcloud",
 		NewFs:       NewFs,
 		Config: func(name string, m configmap.Mapper) {
-			err := oauthutil.Config("pcloud", name, m, oauthConfig)
+			err := oauthutil.Config("pcloud", name, m, oauthConfig, nil)
 			if err != nil {
 				log.Fatalf("Failed to configure token: %v", err)
 			}