* Update: Tested against WooCommerce 3.9.0.
* New: Add woocommerce_free_gift_coupon_apply_coupon_data filter for modifying coupon gift data.
* Fix: Use cart getter to access applied coupons.
* Fix: Only show free gift admin UI for supported coupon types.
* New: Add support for WooCommerce Subscriptions' custom coupon types.