Allow 3rd party modules to perform actions after totals calculation

by returning the storage.post promise, third party modules can perform
additional actions by adding .done/.fail or .always tasks to the request
promise by creating a Javascript mixin for the totals processor.

Author: navarr

app/code/Magento/Checkout/view/frontend/web/js/model/cart/totals-processor/default.js

@@ -38,7 +38,7 @@ define([
             payload.addressInformation['shipping_carrier_code'] = quote.shippingMethod()['carrier_code'];
         }
 
-        storage.post(
+        return storage.post(
             serviceUrl, JSON.stringify(payload), false
         ).done(function (result) {
             var data = {
@@ -96,7 +96,7 @@ define([
             ) {
                 quote.setTotals(cartCache.get('totals'));
             } else {
-                loadFromServer(address);
+                return loadFromServer(address);
             }
         }
     };