[Dovecot] Enable mail logger plugin

[Alexandre Ellert]

I've reworked my configuration and now it works as expected.
Here is what it looks like now :

mail_plugins = quota mail_log notify

protocol lda {
  mail_plugins = $mail_plugins sieve
}

protocol imap {
  mail_plugins = $mail_plugins imap_quota antispam
}

protocol pop3 {
  mail_plugins = $mail_plugins
}

Thanks all for your help and for good explanations.

Alexandre.