[w3c/browser-payment-api] Support for gift cards and discount codes (#145)

An important part of Shopify's checkout is the ability for the user to enter a discount code and or gift card at any point during the checkout (shown below). Once the discount or gift card code is verified the checkout balance is adjusted accordingly.

**Step 1 - shipping**

<img src="https://cloud.githubusercontent.com/assets/2962326/14564424/7deaf7c0-02f4-11e6-87be-a4f37ea2bef1.png" width="300px"/>

**Step 2 - delivery**

<img src="https://cloud.githubusercontent.com/assets/2962326/14564586/3411760a-02f5-11e6-94f3-ce63dbce2985.png" width="300px"/>

**Step 3 - payment**

<img src="https://cloud.githubusercontent.com/assets/2962326/14564591/38c262b8-02f5-11e6-99ae-7d8aa1ea1dbd.png" width="300px"/>

The majority of checkout on the web support gift and promotional codes as well. Examples from **Amazon** and **Nike** below.

<img src="https://cloud.githubusercontent.com/assets/2962326/14652786/eeadbfa4-0643-11e6-9812-fc8c898b9519.png" width="300px"/>

<img src="https://cloud.githubusercontent.com/assets/2962326/14652901/7844096c-0644-11e6-8ec1-d87850d6e3ca.png" width="300px"/>

What is the strategy for accepting gift card and discount codes in the payments api? Will this be the responsibility of the user agent or the payment app? 

There has been [related discussion](https://github.com/w3c/browser-payment-api/issues/1) on the ability for the merchant to require the collection of certain shipping and billing information. This is similar but gift card and discount codes are more complex as they require an additional verification step to be carried out by the merchant. This verification could be accomplished through a callback to the merchant where they would be responsible for any updates.

**Note:** It’s more common to include gift card and discount entry in the payments step of checkout, however, we have experienced better cart conversation rates by making the option available throughout our checkout experience.

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