add '--cookies-export' command-line option

docs/options.md

@@ -19,6 +19,7 @@
     --clear-cache MODULE        Delete cached login sessions, cookies, etc. for
                                 MODULE (ALL to delete everything)
     --cookies FILE              File to load additional cookies from
+    --cookies-export FILE       Export session cookies to FILE
     --cookies-from-browser BROWSER[/DOMAIN][+KEYRING][:PROFILE][::CONTAINER]
                                 Name of the browser to load cookies from, with
                                 optional domain prefixed with '/', keyring name

gallery_dl/option.py

@@ -153,6 +153,11 @@ def build_parser():
         dest="cookies", metavar="FILE", action=ConfigAction,
         help="File to load additional cookies from",
     )
+    general.add_argument(
+        "--cookies-export",
+        dest="cookies-update", metavar="FILE", action=ConfigAction,
+        help="Export session cookies to FILE",
+    )
     general.add_argument(
         "--cookies-from-browser",
         dest="cookies_from_browser",