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/message.py

@@ -20,6 +20,7 @@ class AnymailMessageMixin(object):
 
     def __init__(self, *args, **kwargs):
         self.esp_extra = kwargs.pop('esp_extra', UNSET)
+        self.envelope_sender = kwargs.pop('envelope_sender', UNSET)
         self.metadata = kwargs.pop('metadata', UNSET)
         self.send_at = kwargs.pop('send_at', UNSET)
         self.tags = kwargs.pop('tags', UNSET)