enhc: add plain text email body along with HTML increasing deliverability

[jaredfolkins]

I saw issue #79 and as I need this functionality, I figured I'd offer a PR.

Thanks,
Jared

[knadh]

Excellent! I think we should make this optional though. A plaintext_email = true option in the config would be perfect for now. In the future, it can be a per-campaign option as well.

[jaredfolkins]

Actually, would you consider, email_format = {plain|html|both}?

[jaredfolkins]

In the future, it can be a per-campaign option as well.

Hehe. I actually started here. I have a lot of the code done but then I realized that functions like Push() would most likely need their signatures changed substantially.

[knadh]

email_format = {plain|html|both} is the ideal scenario.

Push() is a bit ugly. It has too many params and should've been refactored earlier. We can introduce a messenger.Message struct and have Push() just take that.

[jaredfolkins]

Please review. Thanks!

[knadh]

Cheers. Will push a release soon with a few other things.