WooCommerce Customer Order Coupon CSV Import Suite 3.12.0

* Tweak - Add option to control whether order emails should be sent during imports
* Fix - Ensure line items with zero-value totals properly imported
* Fix - Ensure correct admin url is used on column mapper form

* Fix - Avoid PHP errors during order import

* Tweak - Try to match line item meta to product attributes if the meta key matches a product attribute label
* Fix - Ensure imported line item product attributes are recognized by WooCommerce when they exist
* Fix - Ensure empty line items are skipped when importing orders
* Misc - Add support for WooCommerce 3.3
* Misc - Remove support for WooCommerce 2.5