Allow multi-level fields for perf/output vars

[DragonQ]

Currently, specifying perf or output vars only works for top-level or simple fields within JSON. It doesn't work with more complicated node trees, e.g. "{main_node}[0]->{secondary_node}[4]->{final_node}".

This change fixes this issue by parsing the perf and output var JSON fields in the same way that the attribute JSON field is parsed.