Support merge_metadata in Mailgun, Mailjet, Mandrill, Postmark,
SparkPost, and Test backends. (SendGrid covered in earlier PR.)
Also:
* Add `merge_metadata` to AnymailMessage, AnymailMessageMixin
* Add `is_batch()` logic to BasePayload, for consistent handling
* Docs
Note: Mailjet implementation switches *all* batch sending from their
"Recipients" field to to the "Messages" array bulk sending option.
This allows an independent payload for each batch recipient.
In addition to supporting merge_metadata, this also removes the
prior limitation on mixing Cc/Bcc with merge_data.
Closes#141.