Provide default value to result method. Fix #1495

This pull request fulfills the proposal suggested in #1495. With this addition, `_.result` now supports:

```javascript
_.result(null, 'method', 'default') // => 'default'
_.result('string'.match('missing'), 1, 'default'); // => 'default'
_.result({}, 'method', 'default') // => default
```

Additionally, I had considered allowing the default value to be a function, which could be fairly useful. However that requires a more significant change and I wanted to first gauge if others found this would be particularly valuable.