Added functionality to unsubscribeFromTopic

Author: experionarjun

android/src/main/java/com/dieam/reactnativepushnotification/modules/RNPushNotification.java

@@ -145,6 +145,11 @@ public void requestPermissions(String senderID) {
     public void subscribeToTopic(String topic) {
         FirebaseMessaging.getInstance().subscribeToTopic(topic);
     }
+    
+    @ReactMethod
+    public void unsubscribeFromTopic(String topic) {
+        FirebaseMessaging.getInstance().unsubscribeFromTopic(topic);
+    }
 
     @ReactMethod
     public void presentLocalNotification(ReadableMap details) {

index.js

@@ -280,6 +280,10 @@ Notifications.subscribeToTopic = function() {
 	return this.callNative('subscribeToTopic', arguments);
 };
 
+Notifications.unsubscribeFromTopic = function () {
+	return this.callNative('unsubscribeFromTopic', arguments);
+};
+
 Notifications.presentLocalNotification = function() {
 	return this.callNative('presentLocalNotification', arguments);
 };