SMTP 配置支持腾讯企业邮箱接口

现在 smtp 看起来只传了 地址 和 端口，腾讯的还需帐号，类似:

```
mail.backend: 'smtp'  # Use dummy if you want to disable email entirely
mail.host: 'smtp.exmail.qq.com'
mail.port: 25
mail.username: 'XXXX@X.X'
mail.password: 'XXXX'
mail.use-tls: false
# The email address to send on behalf of
mail.from: 'XXXX@X.X'
mail.list-namespace: 'X.X'
```
