Add attachments to newsletter message

[JungDev]

No description provided.

[dokterbob]

Hey @JungDev,

Thanks for the contrib! If you'd like to have your code included in master, there are a couple of things you have to take care of though:

1. Make sure the tests are passing. In any case, Travis should report passing tests.
2. Extended tests to cover any additional code included in your commit. In any case, the coveralls report should report increased coverage (when functionality has been added).
3. Make a clear description of the added functionality, including the why and how. Make sure the way you implemented the functionality is generic enough for other users to make use of and does not degrade the performance of existing users.

[frennkie]

@dokterbob I would really appreciate if it would be possible to add attachments to the newsletters that are send out.

You listed three points that need to be added/improved to get this PR merged. If I start working on them - how are the chances to get this merged into django-newsletter?

@JungDev I know almost 5 years is a long time.. but did you ever continue working on or at least using the code from your PR?

[dokterbob]

I'm open to it but will have limited time to give feedback and/or review. Perhaps any of the other contributors are able to help? @dsanders11 @claudep @pcraston