add param, allowing an 'includes' array

Sometimes it's nicer to specify the options we'd like to include.
The change is really minor but here's good example why it's useful:
```javascript
var options = {
  apple: 'valueA',
  bread: 'valueB',
  carrot: 'valueC',
  dairy: 'valueD',
  eggs: 'valueE',
  figs: 'valueF',
  grapes: 'valueG',
};
// can now do this:
var args = dargs(options, [], ['apple', 'bread'])

// instead of this:
var args = dargs(options, ['carrot', 'dairy', 'eggs', 'figs', 'grapes'])
```
besides a few less characters, sometimes it's easier to specify what you need, instead of figuring out what you don't need.