allow disabling starttls

- use more up-to-date gomail fork
- add starttls option

DOCS.md

@@ -10,6 +10,7 @@ You can configure the plugin using the following parameters:
 * **username** - SMTP username
 * **password** - SMTP password
 * **skip_verify** - Skip verification of SSL certificates, defaults to `false`
+* **starttls** - Enable/Disable STARTTLS
 * **recipients** - List of recipients to send this mail to (besides the commit author)
 * **recipients_file** - Filename to load additional recipients from (textfile with one email per line) (besides the commit author)
 * **recipients_only** - Do not send mails to the commit author, but only to **recipients**, defaults to `false`
@@ -127,3 +128,22 @@ steps:
       password: 12345
 +     skip_verify: true
 ```
+
+### STARTTLS
+
+By default, STARTTLS is being used opportunistically meaning, if advertised
+by the server, traffic is going to be encrypted.
+
+You may want to disable STARTTLS, e.g., with faulty and/or internal servers:
+
+```diff
+steps:
+  - name: notify
+    image: drillster/drone-email
+    settings:
+      from: noreply@github.com
+      host: smtp.mailgun.org
+      username: octocat
+      password: 12345
++     starttls: false
+```