Fix passing -i/--index to sub command

cmd/msgbus/sub.go

@@ -58,6 +58,9 @@ func init() {
 		"index", "i", -1,
 		"position in the topic's sequence to start subscribing from (-1 indicates end)",
 	)
+
+	viper.BindPFlag("index", subCmd.Flags().Lookup("index"))
+	viper.SetDefault("index", -1)
 }
 
 func handler(command string, args []string) msgbus.HandlerFunc {