implement 'archive-pragma' option

docs/gallery-dl-example.conf

@@ -5,6 +5,7 @@
 
         "#": "set global archive file for all extractors",
         "archive": "~/gallery-dl/archive.sqlite3",
+        "archive-pragma": ["journal_mode=WAL", "synchronous=NORMAL"],
 
         "#": "add two custom keywords into the metadata dictionary",
         "#": "these can be used to further refine your output directories or filenames",
@@ -36,7 +37,7 @@
 
         "pixiv":
         {
-            "#": "override global archive setting for pixiv",
+            "#": "override global archive path for pixiv",
             "archive": "~/gallery-dl/archive-pixiv.sqlite3",
 
             "#": "set custom directory and filename format strings for all pixiv downloads",