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.

The current commit add the field to the invoice object, but it's always set
to the current customer email to keep the implementation simple.

[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