src/riemann-client.ronn: Force markdown-mode

Add a comment at the top, to help Emacs realize this is in Markdown.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
src/riemann-client.c: Handle optional properies with JSON output

When using the JSON output mode, don't choke on missing properties.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>