Hook for dynamic mail headers

[dokterbob]

Either a dictionary or a callable returning a dictionary, configured in the Django settings, should allow for dynamically injecting headers into the SMTP request for (statistics) processing, such as with Sendgrid (see django-sendgrid-events for example).

The resulting API should be rich enough to allow for processing of individual newsletter submissions into the API. Actually reporting this information back in the admin interface (but only when reporting is enabled) could be a second ticket related to this.