pass file through

I wanted to make this work with `gulp-livereload`, but since the callback is passed `null`, it was failing.

Since it doesn't really matter what we give this plugin, you can do something like this now:

```
gulp.src('_site/index.html')
  .pipe(jekyll())
  .pipe(connect.livereload());
```

...which tells the livereload server of `gulp-connect` that `_site/index.html` was updated.