Fix for evancohen/issues/65, extracting argumnets from arrow function…

… behaves erratically

lib/annyang-core.js

@@ -30,8 +30,7 @@ const commandToRegExp = function (command) {
 };
 
 // This method receives an array of callbacks to iterate over, and invokes each of them
-const invokeCallbacks = callbacks => {
-    let args = Array.prototype.slice.call(arguments, 1);
+const invokeCallbacks = (callbacks, ...args) => {
     callbacks.forEach(callback =>
         callback.callback.apply(callback.context, args)
     );