riemann: Add support for $SEQNUM

Supporting $SEQNUM is fairly easy, and in case of the Riemann
destination, mighty useful too, so lets add that.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
riemann: Minor formatting fixup

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>