Support email notification without SMTP auth.
Support email notification without SMTP auth.

Even if .drone.yml does not specify username and password,
gomail.NewPlainDialer is still populated with them - albeit empty -
which means they are still being sent to SMTP server. In case of
Google's smtp-relay.gmail.com this will result in an error, even
if CI's server IP address is added to exclusion list allowing it to
skip auth entirely.

This commit constructs gomail.Dialer without username and password
if there was no username found in .drone.yml.

* Constructs gomail.Dialer without username when no username in .drone.yml
* Replaces gomail.NewPlainDialer with gomail.NewDialer

Drillster/drone-email,drone-plugins/drone-email,igoraj/drone-email