Add ESP templates, batch send and merge

* message.template_id to use ESP stored templates
* message.merge_data and merge_global_data
  to supply per-recipient/global merge variables
  (with or without an ESP stored template)
* When using per-recipient merge_data, tell ESP to use
  batch send: individual message per "to" address.
  (Mailgun does this automatically; SendGrid requires
  using a different "to" field; Mandrill requires
  `preserve_recipients=False`; Postmark doesn't
  support *this type* of batch sending with merge data.)
* Allow message.from_email=None (must be set after
  init) and message.subject=None to suppress those
  fields in API calls (for ESPs that allow "From" and
  "Subject" in their template definitions).

Mailgun:
* Emulate merge_global_data by copying to
  recipient-variables for each recipient.

SendGrid:
* Add delimiters to merge field names via
  esp_extra['merge_field_format'] or
  ANYMAIL_SENDGRID_MERGE_FIELD_FORMAT setting.

Mandrill:
* Remove Djrill versions of these features;
  update migration notes.

Closes #5.

docs/esps/postmark.rst

@@ -115,6 +115,52 @@ see :ref:`unsupported-features`.
   Postmark does not support :attr:`~anymail.message.AnymailMessage.send_at`.
 
 
+.. _postmark-templates:
+
+Batch sending/merge and ESP templates
+-------------------------------------
+
+Postmark supports :ref:`ESP stored templates <esp-stored-templates>`
+populated with global merge data for all recipients, but does not
+offer :ref:`batch sending <batch-send>` with per-recipient merge data.
+Anymail's :attr:`~anymail.message.AnymailMessage.merge_data`
+message attribute is not supported with the Postmark backend.
+
+To use a Postmark template, set the message's
+:attr:`~anymail.message.AnymailMessage.template_id` to the numeric
+Postmark "TemplateID" and supply the "TemplateModel" using
+the :attr:`~anymail.message.AnymailMessage.merge_global_data`
+message attribute:
+
+  .. code-block:: python
+
+      message = EmailMessage(
+          ...
+          subject=None,  # use template subject
+          to=["alice@example.com"]  # single recipient...
+          # ...multiple to emails would all get the same message
+          # (and would all see each other's emails in the "to" header)
+      )
+      message.template_id = 80801  # use this Postmark template
+      message.merge_global_data = {
+          'name': "Alice",
+          'order_no': "12345",
+          'ship_date': "May 15",
+          'items': [
+              {'product': "Widget", 'price': "9.99"},
+              {'product': "Gadget", 'price': "17.99"},
+          ],
+      }
+
+Set the EmailMessage's subject to `None` to use the subject from
+your Postmark template, or supply a subject with the message to override
+the template value.
+
+See this `Postmark blog post on templates`_ for more information.
+
+.. _Postmark blog post on templates:
+    https://postmarkapp.com/blog/special-delivery-postmark-templates
+
 
 .. _postmark-webhooks: