Return the SMTP server response when sending an e-mail

[l0ck3]

Adresses #121

The Bamboo.SMTPAdapter.deliver/1 now returns the raw response of the SMTP server when an e-mail is sent successfully.

Previously: :ok
Now: {:ok, <response>}

⚠️ Requires a major version bump since it breaks the existing API.

[kdisneur]

If we want to use this feature, we need to add an extra response: true parameter to Bamboo as explained here. Otherwise Bamboo won't forward the response from the adapter:

BambooTrial.Mailer.deliver_now(BambooTrial.Email.test_email(), response: true)

# => {%Bamboo.Email{....}, {:ok, "Ok 0102016cd2444445-1a01c99c-6dc3-4c67-b703-11e000b9d642-000000\r\n"}}