Add the ability to add to the list of tags and attributes in an event

Instead of only being able to replace the list of tags and attributes
within an event, make it possible to add to the list too.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
src/riemann-client: Add a --ttl option

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