[pp] allow inheriting settings from global 'postprocessor' entries

No idea how to properly explain/document this, so here's an example:

The extractor.postprocessors object
gets its options from postprocessor.jl
and adds 'filename' itself.

{
    "extractor": {
        "postprocessors": {
            "type": "jl",
            "filename": "meta.jsonl"
        }
    },

    "postprocessor": {
        "jl": {
            "name": "metadata",
            "mode": "jsonl",
            "open": "a"
        }
    }
}

gallery_dl/job.py

@@ -622,6 +622,14 @@ class DownloadJob(Job):
             for pp_dict in postprocessors:
                 if isinstance(pp_dict, str):
                     pp_dict = pp_conf.get(pp_dict) or {"name": pp_dict}
+                elif "type" in pp_dict:
+                    pp_type = pp_dict["type"]
+                    if pp_type in pp_conf:
+                        pp = pp_conf[pp_type].copy()
+                        pp.update(pp_dict)
+                        pp_dict = pp
+                    if "name" not in pp_dict:
+                        pp_dict["name"] = pp_type
                 if pp_opts:
                     pp_dict = pp_dict.copy()
                     pp_dict.update(pp_opts)