[options] add '-a' as short option for '--user-agent'

docs/options.md

@@ -11,7 +11,7 @@
     -d, --destination PATH      Target location for file downloads
     -D, --directory PATH        Exact location for file downloads
     -X, --extractors PATH       Load external extractors from PATH
-    --user-agent UA             User-Agent request header
+    -a, --user-agent UA         User-Agent request header
     --clear-cache MODULE        Delete cached login sessions, cookies, etc. for
                                 MODULE (ALL to delete everything)
     --compat                    Restore legacy 'category' names

gallery_dl/option.py

@@ -276,7 +276,7 @@ def build_parser():
         help="Load external extractors from PATH",
     )
     general.add_argument(
-        "--user-agent",
+        "-a", "--user-agent",
         dest="user-agent", metavar="UA", action=ConfigAction,
         help="User-Agent request header",
     )