Feature: Add envelope_sender

New EmailMessage attribute `envelope_sender` controls ESP's sender,
sending domain, or return path where supported:

* Mailgun: overrides SENDER_DOMAIN on individual message
  (domain portion only)
* Mailjet: becomes `Sender` API param
* Mandrill: becomes `return_path_domain` API param
  (domain portion only)
* SparkPost: becomes `return_path` API param
* Other ESPs: not believed to be supported

Also support undocumented Django SMTP backend behavior, where envelope
sender is given by `message.from_email` when
`message.extra_headers["From"]` is set. Fixes #91.

tests/test_sendinblue_backend.py

@@ -270,6 +270,12 @@ class SendinBlueBackendStandardEmailTests(SendinBlueBackendMockAPITestCase):
 class SendinBlueBackendAnymailFeatureTests(SendinBlueBackendMockAPITestCase):
     """Test backend support for Anymail added features"""
 
+    def test_envelope_sender(self):
+        # SendinBlue does not have a way to change envelope sender.
+        self.message.envelope_sender = "anything@bounces.example.com"
+        with self.assertRaisesMessage(AnymailUnsupportedFeature, 'envelope_sender'):
+            self.message.send()
+
     def test_metadata(self):
         self.message.metadata = {'user_id': "12345", 'items': 6, 'float': 98.6, 'long': longtype(123)}
         self.message.send()