message: Add clientSigningSecret to SlackConfiguration
  Resolves
diff: @@ -303,6 +303,7 @@ declare namespace botkit {
  api_root?: string;
  clientId?: string;
  clientSecret?: string;
  + clientSigningSecret?: string;
  disable_startup_messages?: boolean;
  incoming_webhook?: { url: string; };
  interactive_replies?: boolean;
  