proposal: sendgrid support

[lopezator]

What do you think about supporting Sendgrid?

In my case that's the tool we are using internally for our mailing, so being able to deploy our own burner.kiwi instance but using sendgrid would be a great plus.

[haydenwoodhead]

So I don't really have the time to dedicate to adding it. But if you're interested in writing this implementation yourself I'm more than happy to help. You just have to implement an interface https://github.com/haydenwoodhead/burner.kiwi/blob/master/email/provider.go in a similar way to mailgun.

[haydenwoodhead]

burner.kiwi can now receive it's own email with built in SMTP support.