feat(cumulativedeferred): Added subDefer method to CumulativeDeferred

ts/smartpromise.classes.cumulativedeferred.ts

@@ -15,6 +15,12 @@ export class CumulativeDeferred {
     }, 0);
   }
 
+  public subDefer() {
+    const done = defer();
+    this.addPromise(done.promise);
+    return done;
+  }
+
   public addPromise(promiseArg: Promise<any>) {
     this.accumulatedPromises.push(promiseArg);
   }