=head2 anykeyword

This module requires that every incoming message have at least one keyword
on the subject line.  For the purposes of this check, a keyword is defined
as anything that matches:

    /^(?:Re:\s+)?\[\S+\]/

(in other words, something without spaces enclosed in square brackets at
the beginning of the subject line modulo any Re:).  If no keyword is found
in the incoming message, this module returns the message:

    No keyword found

This module takes no arguments or configuration directives.