Support delete invoice endpoint

[ktravers]

Endpoint: DELETE /v1/invoices/{INVOICE_ID}
Docs: https://stripe.com/docs/api/invoices/delete

This endpoint is part of a suite of new invoice API updates introduced in version 2018-11-08.

In order to support this endpoint, the stripity-stripe library most likely will need to be updated to the latest API version 2018-11-08, which I understand is well outside the scope of this ticket. Happy to close / revise this ticket as necessary.