require array, corresponding to browserify -r option

Example:

```javascript
browserify: {
  bundle: {
    src: [],
    require: ['mongodb', 'tcp_wrap-chromeify'],
    dest: 'app/scripts/vendor/bundle.js',
    ignore: 'node_modules/browser-resolve/node_modules/buffer-browserify/index.js',
    options: {
      globals: {
        process: 'browser-resolve/builtin/process',
        Buffer: 'buffer-browserify'
      }
    }
  }
}
```