Update shipping option

Setting list of ShippingOptions like in payments_example.py
shipping_options = [
    ShippingOption(id='instant', title='WorldWide Teleporter').add_price(LabeledPrice('Teleporter', 1000)),
    ShippingOption(id='pickup', title='Local pickup').add_price(LabeledPrice('Pickup', 300))]
gives us [None,None],so It's better add_price to return self

telebot/types.py

@@ -1902,6 +1902,7 @@ class ShippingOption(JsonSerializable):
         """
         for price in args:
             self.prices.append(price)
+        return self
 
     def to_json(self):
         price_list = []