Subscriptions Gifting support

[petertwise]

I've created a fork to support the Gifting for Woo Subscriptions add-on plugin.

https://github.com/squarecandy/woocommerce-subscriptions-importer-exporter

I have it fully functioning, but it would probably take a bit more work to get the code in shape for a pull request, and also it would require some additional documentation write up to be useful to others.

Let me know if there's any interest in adding this to your main woocommerce-subscriptions-importer-exporter code base, or if it's better to just keep my fork separate.

Thanks!