Add missed css_class to TabHolder template

[kosdmit]

PR to fix Issue #178

Now we can use css_class TabHolder attribute to add css classes to html element. For example:
adding "justify-content-center" css class to TabHolder gives this result:

<form method="post">
      <ul class="nav nav-tabs justify-content-center">
        <li class="nav-item">
          <a class="nav-link active" href="#custom-name" data-bs-toggle="tab"
            >One</a
          >
        </li>
        <li class="nav-item">
          <a class="nav-link" href="#two" data-bs-toggle="tab">Two</a>
        </li>
      </ul>
...

[smithdc1]

Thanks @kosdmit -- I pushed a change to the tests. If you could let me know that you are happy with these I can then merge this.

[kosdmit]

If you could let me know that you are happy with these I can then merge this.

I think it's done well, we can do the merge.