attach help and showHelp arguments to the argv store

lib/nconf/stores/argv.js

@@ -61,6 +61,9 @@ Argv.prototype.loadArgv = function () {
     self.set(key, argv[key]);
   });
 
+  this.showHelp = optimist.showHelp
+  this.help     = optimist.help
+
   this.readOnly = true;
   return this.store;
 };