Do not work filter in HTMLParser.

I made search index of this html file.
```html
<!DOCTYPE html>
<html>
  <head></head>
  <body>
    <div class="body">
      <pre>あいうえお</pre>
    </div>
    <div>かきくけこ</div>
  </body>
</html>
```

```sh
./bin/oktavia-mkindex-cli -i ../hoge/ -r ../hoge/ -m html -u file -f .body -c 5 -t web
```

I had specified the filter `-f .body`. But it hits when I search for 'かき'.