Add deprecation warning for omitting --host

router.go

@@ -87,6 +87,8 @@ func StartMainServer(addr string, db *db.Database, fs *db.FileStore, rawhost *st
 		if err != nil {
 			log.Fatalln(errors.Wrap(err, "could not parse host flag"))
 		}
+	} else {
+		log.Println("warning: the --host flag will be required in the future")
 	}
 	rl := rushlink{
 		db:   db,