message: Fix signature of spawn method
  According to documentation at config parameter of spawn() is optional.
diff: @@ -1032,7 +1032,7 @@ export class Botkit {
  * for handling platform-specific events or activities.
  * @param config {any} Preferably receives a DialogContext, though can also receive a TurnContext. If excluded, must call `bot.changeContext(reference)` before calling any other method.
  */
  - public async spawn(config: any): Promise<BotWorker> {
  + public async spawn(config?: any): Promise<BotWorker> {
  if (config instanceof TurnContext) {
  config = {
  dialogContext: await this.dialogSet.createContext(config as TurnContext),
  