add '--version'

gallery_dl/__init__.py

@@ -58,6 +58,10 @@ def build_cmdline_parser():
         "--list-keywords", dest="list_keywords", action="store_true",
         help="print a list of available keywords for the given URLs",
     )
+    parser.add_argument(
+        "--version", action="version", version=__version__,
+        help="print program version and exit"
+    )
     parser.add_argument(
         "urls",
         nargs="*", metavar="URL",