Support additional options in Django email backend

[richleland]

We recently added some code related to #107 to allow Django users to set template and substitution_data. It would be nice if we had a simple way to allow other options without having to modify the code, add tests, etc (think campaign, click and open tracking, etc.).

[richleland]

Closing as we're looking at pointing people to django-anymail in 2.0