unpaid subscription status undocumented

[ba32107]

In dev mode, I can set an unpaid status:

This status is not present in the main README or this doc:
https://github.com/Glench/ExtPay/blob/main/docs/how_subscriptions_work.md

It's not really clear what the difference is between past_due and unpaid. The Stripe docs say:

If subscription collection_method=send_invoice it becomes past_due when its invoice is not paid by the due date, and canceled or unpaid if it is still not paid by an additional deadline after that.

From this I can sort of understand the difference, but it's never mentioned anywhere in the ExtPay docs that the subscriptionStatus property mimicks the Stripe API's values. So I think this additional value should be documented in this repo.

Thanks!

[Glench]

Good call! I made an issue to update this. #223