accumulate postprocessor objects (#994)

Instead of one 'postprocessors' setting overwriting all others lower
in the hierarchy, all postprocessors along the config path will now
get collected into one big list.

For example '--mtime-from-date' will therefore no longer cause
other postprocessor settings in a config file to get ignored.

gallery_dl/extractor/common.py

@@ -72,6 +72,10 @@ class Extractor():
         return config.interpolate(
             ("extractor", self.category, self.subcategory), key, default)
 
+    def config_accumulate(self, key):
+        return config.accumulate(
+            ("extractor", self.category, self.subcategory), key)
+
     def request(self, url, *, method="GET", session=None, retries=None,
                 encoding=None, fatal=True, notfound=None, **kwargs):
         tries = 1