JSHint --extract option added

How to use it:

```javascript
gulp.task('lint', function () {
  return gulp.src('index.html')
    .pipe(jshint.extract('auto'))
    .pipe(jshint())
    .pipe(jshint.reporter('default'))
});
```
Arguments are auto | always | never, set to always by default. The option needs to be passed before the jshint() pipe.