This makes it possible to specify 'mode' and/or 'event' options of a
postprocessor in its 'name' as
"NAME/MODE@EVENT" or "NAME/MODE" or "NAME@EVENT"
For example
"postprocessors": "metadata/jsonl@file,skip"
is equivalent to
"postprocessors": {
"name" : "metadata",
"mode" : "jsonl",
"event": ["file", "skip"]
}