[config] support accumulating non-list values

fixes 1264fc518b

gallery_dl/config.py

@@ -261,13 +261,19 @@ def accumulate(path, key, conf=_config):
         if key in conf:
             value = conf[key]
             if value:
-                result.extend(value)
+                if isinstance(value, list):
+                    result.extend(value)
+                else:
+                    result.append(value)
         for p in path:
             conf = conf[p]
             if key in conf:
                 value = conf[key]
                 if value:
-                    result[:0] = value
+                    if isinstance(value, list):
+                        result[:0] = value
+                    else:
+                        result.insert(0, value)
     except Exception:
         pass
     return result

gallery_dl/job.py

@@ -619,8 +619,6 @@ class DownloadJob(Job):
             pp_opts = cfg("postprocessor-options")
             pp_list = []
 
-            if isinstance(postprocessors, (dict, str)):
-                postprocessors = (postprocessors,)
             for pp_dict in postprocessors:
                 if isinstance(pp_dict, str):
                     pp_dict = pp_conf.get(pp_dict) or {"name": pp_dict}