Allow for skip processing of listed Message-Id but keep counting

[lukas-vlcek]

Similarly to #12 we need option to exclude message-ids from processing but keep counting. Typically this involve SPAM messages that haven't been removed by mail list admin and they are still part of public mail list archive.

We can easily identify such stuff using search (like "viagra") and then exclude these messages from indexing.

(There is bunch of such content in jbossws-dev and jbossws-users mail lists)