Need generic "admonition" directive with `:class` support

[weaverryan]

Here's something we have currently:

.. admonition:: Screencast
    :class: screencast

    Do you prefer video tutorials? Check out the `Joyful Development with Symfony`_
    screencast series.

The current HTML is:

<div class="screencast admonition-wrapper">
    <div class="admonition admonition-">
        <p class="first admonition-title">Screencast</p>
        <p class="last">Do you prefer video tutorials? Check out the <a class="reference external" href="http://symfonycasts.com/screencast/symfony">Stellar Development with Symfony</a>
screencast series.</p>
    </div>
</div>

We have the AbstractAdmonitionDirective, but I don't think it supports the "name" being an option passed to the directive and I also don't think it supports the :class: option

[weaverryan]

This has been added at sha: b4f5b18 and support for the :class: option was added to all admonitions in sha: 0a95061