undocumented lightly tested support for CODE ref filters

undocumented lightly tested support for CODE ref filters

For 11 years the simple filtering has been fine, but filtering of code review
mails at work finally requires something more complex quick example:

@FILTERS = (
  # Prioritize reviews to/cc me in special folder.
  sub {
      my $h = shift;
      match($h, ' TO', ['main-review-list', 't'])
          && match($h, ' TO', ['me', 't'])
          && return 'reviews';
  },
  # Isolate the spammy reviews.
  [' TO', ['hi-traffic-review-list', 'wharrgarbl']],

  # Other reviews follow normal rules below.
);