scl: Add a graphite() SCL

To make it easier to send metrics to Graphite, the graphite()
destination block will set up a pre-configured network destination, with
$(graphite-output) embedded. By default, the payload is empty, and one
must set the options passed to $(graphite-output) when using the SCL
block.

This closes #65.

Requested-by: Balazs Scheidler <bazsi@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>