Add warnings flag

This commit adds a warnings flag

It can be combined with -q / --quiet to display warnings.
The intent is to provide a silent option that still surfaces
warning and error messages so that they are visible in logs.

gallery_dl/option.py

@@ -255,6 +255,12 @@ def build_parser():
         action="store_const", const=logging.DEBUG,
         help="Print various debugging information",
     )
+    output.add_argument(
+        "-w", "--warning",
+        dest="loglevel",
+        action="store_const", const=logging.WARNING,
+        help="Print warnings",
+    )
     output.add_argument(
         "-g", "--get-urls",
         dest="list_urls", action="count",