need clear all subscription method

[rupeshtiwari]

Hi
In our angular test cases project, we are using amplifyJS and each time we inject ampilfyjs we want to clear all of its previous subscriptions. Since it is singleton service in angular project this function is called once and all subscriptions remains.
If you check any pubsub library it is common practice to have clearAllSubscriptions method. Therefore, its good idea to put it in. I am willing to submit PR for this change and I hope it is accepted.