add a config switch to archive mails

[stefan-walluhn]

postoffice creates a maildir in the form .YYYY.MM.DD inside the maildir folder to archive processed mails. Since this behavior blows up the incoming maildir a lot it would be nice to have a config switch whether or not postoffice should create this mail archives.

[tseaver]

I would be glad to review and merge a pull request implementing such a switch, but don't have need for it myself (keeping the mails around "forever" is a hard requirement in my deployments).