In docs install examples, show double quotes around
package specifiers that include square brackets, to
prevent them from being interpreted as shell globs.
(Helps with installation on Windows and zsh, e.g.)
Closes#188
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.
