message: Fix usePlugin parameter types declaration
diff: @@ -482,7 +482,7 @@ export class Botkit {
  * Load a plugin module and bind all included middlewares to their respective endpoints.
  * @param plugin_or_function A plugin module in the form of function(botkit) {...} that returns {name, middlewares, init} or an object in the same form.
  */
  - public usePlugin(plugin_or_function: (botkit: Botkit) => BotkitPlugin | BotkitPlugin): void {
  + public usePlugin(plugin_or_function: ((botkit: Botkit) => BotkitPlugin) | BotkitPlugin): void {
  let plugin: BotkitPlugin;
  if (typeof (plugin_or_function) === 'function') {
  plugin = plugin_or_function(this);
  