implement '--clear-cache'

Effectively clears all cached values from the cache database by
executing "DELETE FROM data" without any further user input.

gallery_dl/option.py

@@ -93,6 +93,11 @@ def build_parser():
         metavar="URL", action=ConfigAction, dest="proxy",
         help="Use the specified proxy",
     )
+    general.add_argument(
+        "--clear-cache",
+        dest="clear_cache", action="store_true",
+        help="Delete all cached login sessions, cookies, etc.",
+    )
 
     output = parser.add_argument_group("Output Options")
     output.add_argument(