Use force option with a jshintrc file

When i specified a jshintrc file, it takes options from this file, just as expected. But the problem is the option `force` is not in my jshintrc, because it's not a valid jshint option, so there is no way to pass `force` option.

When i build using grunt, the force option is not present and my task doesn't fail when there is some linting errors.

I have fixed this with an extend of options instead of a replacement.