Adds a mailfilter repository setting.

The mailfilter repository setting allows to define a pipe-style 
email processor, which is able to verify and/or to process emails.

The process will be called on each email, which is to be synced. It
may alter the email by just printing a different mail (headers and 
body), or cancel this message from being processed further.

"mailfilter" is a repository level setting; the mailfilter is used
when syncing *to* the specified repository.
Add arguments to mailfilter.

The mailfilter command will no longer be called via the shell, and
receives a number of arguments, as follows:

    mailfilter <src-repo> <src-folder> <dest-repo> <dest-folder>