* 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.
* Add filename param to attach_inline_image
* Add attach_inline_image_file function
(parallels EmailMessage.attach and attach_file)
* Use `Content-Disposition: inline` to decide
whether an attachment should be handled inline
(whether or not it's an image, and whether or not
it has a Content-ID)
* Stop conflating filename and Content-ID, for
ESPs that allow both. (Solves problem where
Google Inbox was displaying inline images
as attachments when sent through SendGrid.)
