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.

anymail/backends/mailjet.py

@@ -221,6 +221,9 @@ class MailjetPayload(RequestsPayload):
             "content": attachment.b64content
         })
 
+    def set_envelope_sender(self, email):
+        self.data["Sender"] = email.addr_spec  # ??? v3 docs unclear
+
     def set_metadata(self, metadata):
         # Mailjet expects a single string payload
         self.data["Mj-EventPayLoad"] = self.serialize_json(metadata)