code: function npmInstall(packages, callback) {
    callback = utils.defineCallback(callback);
    if (_.isString(packages)) {
      packages = [ packages ];
    }
    return npm.load({
      loglevel: "silent",
      prefix: tmpDir,
    }, function(loadErr) {
      if (loadErr) {
        return callback(new errors.NpmLoadError(loadErr));
      }
      npm.commands.install(packages, function(installErr) {
        if (installErr) {
          return callback(new errors.NpmInstallError(installErr));
        }
        ncp(tmpDir + "/node_modules/", pluginsDir, function(copyError) {
          if (copyError) {
            return callback(new errors.PluginsMoveFromTempError(copyError));
          }
          return callback(null);
        }); // ncp
      }); // npm.commands.install
    }); // npm.load
  }
docs: Install plugins using NPM
  
  @param  {String|String[]} packages - name of packages
  @param  {Function} [callback]