Queue hash private methods support

This PR adds private methods (underscored methods) support for queue hashes.

That means we can write code like this:

```js
writing: {
  executed: function() {
    this._private();
  },
  _private: function() {
    console.log('I will not be executed by the generator until someone calls me');
  }
}
```

*Note: I'm not familiar with Yeoman. There's maybe a way to get queue hash private methods support without this fix. I just didn't find anything in the documentation.*

Yeoman is awesome :heart: