output help only when there are no arguments

knex cli should output help only when there are no arguments.
current implementation always outputs help, even with correct arguments, such as `knex migrate:status`
the argument length should be checked via process.argv.
reference: https://github.com/tj/commander.js/#outputhelpcb

bin/cli.js

@@ -341,7 +341,7 @@ function invoke(env) {
         .catch(exit);
     });
 
-  if (!commander._args.length) {
+  if (!process.argv.slice(2).length) {
     commander.outputHelp();
   }