.jshintrc should extend options, not override

If specify `jshintrc` option in `options`, then it override all another options.

For example:
```javascript
grunt.initConfig({
  jshint: {
    options: {
      jshintrc: ".jshintrc",
      node: true
    }, 
    all: {
      src: ['src/**/*.js']
    }
  }
});
```

`node` wouldn't be set.

This patch fixes this.