Implements the %module command, that can be used to install a specific named module, using npm. The module gets installed into 'node_modules' within the path specified in configuration.
function moduleCommand(shell, args, data, evaluationId) {
  var deferred = shell.runtime.q.defer();

  installer.install(args.name, shell.config.userPath, /* quiet */ false, function(error) {
    if (error) {
      deferred.reject(shell.createError('Could not install module'));
    }
    else {
      shell.installedModules[args.name] = true;
      deferred.resolve();
    }
  });

  return deferred.promise;
}