Add optional context to _.result

I added an optional third `context` parameter to `_.result`. This is useful for when you want to use `_.result` with a prototype object.

I'm using it to extend the `events` object in a `Backbone.View` like this:
```
var BaseView = Backbone.View.extend({
  events: { 'click': 'onClick' }
});

var ExtendedView = BaseView.extend({
  events: function() {
    return _.extend({
      'click .something-else': 'onClickSomethingElse'
    }, _.result(BaseView.prototype, 'events', this));
  }
});
```