Invoice: Add customer_email in invoice object

[Stripe documentation on invoices] describes this field like this:

> customer_email - nullable string
>
> The customer’s email. Until the invoice is finalized, this field will equal
> customer.email. Once the invoice is finalized, this field will no longer be
> updated.

This is the field that should be used to know where an invoice was effectively
sent (when using Stripe or a third-party solution to send invoices by email)

Note that the current implementation only updates the field on invoice
finalization to keep the implementation simple.

[Stripe documentation on invoices]:
https://docs.stripe.com/api/invoices/object#invoice_object-customer_email

Author: feliixx

localstripe/resources.py

@@ -1208,6 +1208,7 @@ def __init__(self, customer=None, subscription=None, metadata=None,
         super().__init__()
 
         self.customer = customer
+        self.customer_email = cus.email
         self.subscription = subscription
         self.tax_percent = tax_percent
         self.default_tax_rates = default_tax_rates
@@ -1334,6 +1335,8 @@ def charge(self):
     def _finalize(self):
         assert self.status == 'draft'
         self._draft = False
+        cus = Customer._api_retrieve(self.customer)
+        self.customer_email = cus.email
         self.status_transitions['finalized_at'] = int(time.time())
 
     def _on_payment_success(self):