feat(save): add 'save' method

src/methods/save.js

@@ -0,0 +1,26 @@
+export default self => {
+  const { cart, storageKey, localStorage } = self
+  // fix cart subtotal first
+  cart.subtotal = 0
+  for (let i = 0; i < cart.items.length; i++) {
+    const item = cart.items[i]
+    cart.subtotal += item.quantity * (item.final_price || item.price)
+  }
+  if (typeof localStorage === 'object' && localStorage) {
+    localStorage.setItem(storageKey, JSON.stringify(cart))
+  }
+  return self
+}
+
+/**
+ * @method
+ * @name EcomCart#save
+ * @description Save cart object to local storage.
+ *
+ * @returns {self}
+ *
+ * @example
+
+cart.save()
+
+ */