Make use of multiple calendars in GCal module optional

The change in PR #211 to make the GCal module use all calendars
writable to the user is not desirable in all situations. At my
organization, I have write access to calendars for conference
rooms, for people's OOO events, etc. that fill my screen,
obscuring the events on my own calendar.

This change puts the behavior in that change behind a new config
flag, `wtf.mods.gcal.multiCalendar`. It defaults to `false`, but
feel free to change that if you think this is behavior that most
users would want (I tend to default towards preserving existing
behavior, in this case from before that change).

gcal/client.go

@@ -48,23 +48,7 @@ func Fetch() (*calendar.Events, error) {
 		return nil, err
 	}
 
-	// Get all user calendars with at the least writing access
-	pageToken := ""
-	var calendarIds []string
-	for {
-		calendarList, err := srv.CalendarList.List().ShowHidden(false).MinAccessRole("writer").PageToken(pageToken).Do()
-		for _, calendarListItem := range calendarList.Items {
-			calendarIds = append(calendarIds, calendarListItem.Id)
-		}
-
-		pageToken = calendarList.NextPageToken
-		if err != nil || pageToken == "" {
-			break
-		}
-	}
-	if err != nil {
-		return nil, err
-	}
+	calendarIds, err := getCalendarIdList(srv)
 
 	// Get calendar events
 	var events calendar.Events
@@ -179,3 +163,33 @@ func saveToken(file string, token *oauth2.Token) {
 
 	json.NewEncoder(f).Encode(token)
 }
+
+func getCalendarIdList(srv *calendar.Service) ([]string, error) {
+	// Return single calendar if settings specify we should
+	if !wtf.Config.UBool("wtf.mods.gcal.multiCalendar", false) {
+		id, err := srv.CalendarList.Get("primary").Do()
+		if err != nil {
+			return nil, err
+		}
+		return []string{id.Id}, nil
+	}
+
+	// Get all user calendars with at the least writing access
+	var calendarIds []string
+	var pageToken string
+	for {
+		calendarList, err := srv.CalendarList.List().ShowHidden(false).MinAccessRole("writer").PageToken(pageToken).Do()
+		if err != nil {
+			return nil, err
+		}
+		for _, calendarListItem := range calendarList.Items {
+			calendarIds = append(calendarIds, calendarListItem.Id)
+		}
+
+		pageToken = calendarList.NextPageToken
+		if pageToken == "" {
+			break
+		}
+	}
+	return calendarIds, nil
+}