add 'archive-prefix' option (#1711)

docs/configuration.rst

@@ -569,6 +569,16 @@ Description
 .. __: https://docs.python.org/3/library/string.html#format-string-syntax
 
 
+extractor.*.archive-prefix
+--------------------------
+Type
+    ``string``
+Default
+    ``"{category}"``
+Description
+    Prefix for archive IDs.
+
+
 extractor.*.postprocessors
 --------------------------
 Type

gallery_dl/util.py

@@ -1076,9 +1076,9 @@ class DownloadArchive():
             # fallback for missing WITHOUT ROWID support (#553)
             self.cursor.execute("CREATE TABLE IF NOT EXISTS archive "
                                 "(entry PRIMARY KEY)")
-
-        self.keygen = (extractor.category + extractor.config(
-            "archive-format", extractor.archive_fmt)
+        self.keygen = (
+            extractor.config("archive-prefix", extractor.category) +
+            extractor.config("archive-format", extractor.archive_fmt)
         ).format_map
 
     def check(self, kwdict):