Allows for for offline mode with refresh token (#24)

auth.go

@@ -34,16 +34,18 @@ type PersistentStore interface {
 // This is not immediately obvious from the documentation.
 // See https://developer.tdameritrade.com/content/authentication-faq
 type Authenticator struct {
-	Store  PersistentStore
-	OAuth2 oauth2.Config
+	Store    PersistentStore
+	OAuth2   oauth2.Config
+	AuthOpts []oauth2.AuthCodeOption
 }
 
 // NewAuthenticator will automatically append @AMER.OAUTHAP to the client ID to save callers hours of frustration.
-func NewAuthenticator(store PersistentStore, oauth2 oauth2.Config) *Authenticator {
+func NewAuthenticator(store PersistentStore, oauth2 oauth2.Config, opts ...oauth2.AuthCodeOption) *Authenticator {
 	oauth2.ClientID = oauth2.ClientID + "@AMER.OAUTHAP"
 	return &Authenticator{
-		Store:  store,
-		OAuth2: oauth2,
+		Store:    store,
+		OAuth2:   oauth2,
+		AuthOpts: opts,
 	}
 }
 
@@ -75,7 +77,7 @@ func (a *Authenticator) StartOAuth2Flow(w http.ResponseWriter, req *http.Request
 		return "", err
 	}
 
-	return a.OAuth2.AuthCodeURL(state), nil
+	return a.OAuth2.AuthCodeURL(state, a.AuthOpts...), nil
 }
 
 // FinishOAuth2Flow finishes authenticating a user returning from TD Ameritrade.
@@ -104,7 +106,7 @@ func (a *Authenticator) FinishOAuth2Flow(ctx context.Context, w http.ResponseWri
 	if state[0] != expectedState {
 		return nil, fmt.Errorf("invalid state. expected: '%v', got '%v'", expectedState, state[0])
 	}
-	token, err := a.OAuth2.Exchange(ctx, code[0])
+	token, err := a.OAuth2.Exchange(ctx, code[0], a.AuthOpts...)
 	if err != nil {
 		return nil, err
 	}