feat(increase-item-qnt): handle 'increaseItemQnt' method

src/methods/increase-item-qnt.js

@@ -0,0 +1,34 @@
+import fixItemQuantity from './../lib/fix-item-quantity'
+
+export default (self, itemId, quantity = 1, save = true) => {
+  // find respective item on list by ID
+  const item = self.cart.items.find(({ _id }) => _id === itemId)
+  if (item) {
+    item.quantity += quantity
+    fixItemQuantity(item)
+    if (save) {
+      self.save()
+    }
+  } else {
+    return null
+  }
+  return item
+}
+
+/**
+ * @method
+ * @name EcomCart#increaseItemQnt
+ * @description Increase quantity of specific item by ID and save cart.
+ *
+ * @param {string} itemId - The unique object ID of item
+ * @param {integer} [quantity=1] - Quantity to increase (can be negative)
+ * @param {boolean} [save=true] - Save cart data
+ *
+ * @returns {object|null} Returns the updated item object or null
+ * when item not found.
+ *
+ * @example
+
+cart.increaseItemQnt('12300000000000000000000f', 3)
+
+ */