Conditionnaly install npm deps

Gruntfile.js

// Programmatically install npm packages.
  function npmInstall(pkgs, cb) {
    var npm = require('npm');

    npm.load({}, function (err) {
      if (err) {
        throw err;
      }

      console.log(chalk.red(
        '>> Installing npm packages `' + pkgs.join(', ') + '`'
      ));

      npm.commands.install(pkgs, function (err) {
        if (err) {
          throw err;
        }
        cb();
      });

      npm.on('log', console.log);
    });
  }

 // Example

  // Check whether `atom-screenshot` is installed,
  // and install if need be.
  grunt.registerTask('install-screenshot', 'Check and install screenshot deps', function () {
    var done = this.async();
    var atomScreenshot;

    try {
      atomScreenshot = require('atom-screenshot');
    }
    catch (err) {
      npmInstall(['atom-screenshot'], done);
    }
    finally {
      atomScreenshot && done();
    }
  });