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.

So it's the field to use to know where the invoice was effectively sent if it
was sent by email after being finalized.

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

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