Add support for exporting discount codes

[tewshi]

Discount code and price rule export work, but some of the columns couldn't find the matched fields.

• Limit Once Per Order
• Max Uses Per Order
• Applies To: Customers Email

*** And I've used ID instead of handle for products and collections.

Please check @d-beck

[d-beck]

Thanks @tewshi I'll send you an export separately that contains some discounts on the test store with those fields set to see if you can see the info. If you can't let me know and we'll take a look

For the collections and products, what's the level of effort required to do a secondary API call to get the handle for the product/collection?