[pp] implement shortcuts for 'mode' and 'event' options

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"]
}

gallery_dl/job.py

@@ -656,7 +656,26 @@ class DownloadJob(Job):
                         clist, negate)(extr):
                     continue
 
-                name = pp_dict.get("name")
+                name = pp_dict.get("name", "")
+                if "__init__" not in pp_dict:
+                    name, sep, event = name.rpartition("@")
+                    if sep:
+                        pp_dict["name"] = name
+                        if "event" not in pp_dict:
+                            pp_dict["event"] = event
+                    else:
+                        name = event
+
+                    name, sep, mode = name.rpartition("/")
+                    if sep:
+                        pp_dict["name"] = name
+                        if "mode" not in pp_dict:
+                            pp_dict["mode"] = mode
+                    else:
+                        name = mode
+
+                    pp_dict["__init__"] = None
+
                 pp_cls = postprocessor.find(name)
                 if not pp_cls:
                     pp_log.warning("module '%s' not found", name)