additional mailalias support

[Rocco83]

Hi,

It would be good to support additional mailalias.
The problem is the refresh of newaliases, that is missing for external definition.
Currently only the included mailalias is subscribed
https://github.com/dhoppe/puppet-postfix/blob/master/manifests/config.pp#L55

The best would be to have a wrapper to define it, as we can define some template.
And/Or, the following may be added
Mailalias <| |> ~> Exec['postfix.newaliases']

I think the first is the best one, but comments are welcome.