Make welcome-message --verbose work like -v

Change-Id: I0455eef17d07c6918a667cfa94675494e68b6e26
diff --git a/jeepyb/cmd/welcome_message.py b/jeepyb/cmd/welcome_message.py
index e031ff7..b791088 100644
--- a/jeepyb/cmd/welcome_message.py
+++ b/jeepyb/cmd/welcome_message.py
@@ -136,7 +136,7 @@
     # Don't actually post the message
     parser.add_argument('--dryrun', dest='dryrun', action='store_true')
     parser.add_argument('--no-dryrun', dest='dryrun', action='store_false')
-    parser.add_argument('-v', dest='verbose', action='store_true',
+    parser.add_argument('-v', '--verbose', dest='verbose', action='store_true',
                         help='verbose output')
     parser.set_defaults(dryrun=False)