Handle codehilite elements with multiple classes

[PIG208]

It is totally possible for Pygments to generate HTML elements with more than one classes.

This is considered quite an edge case and only happens to some languages, such as PostgreSQL (see pygments/pygments#2780). It currently renders as an unimplemented feature whenever a code block contains tokens with more than one classes.

See a minimal example here.

HTML of the unsupported message:

<pre>
  <span></span>
  <code>
    <span class="s s-Name">"</span>
  </code>
</pre>

[PIG208]

Listed under #921.

[gnprice]

Other than PostgreSQL, this apparently happens with YAML. Here's a test message.

[PIG208]

For these kind of messages, we should be able to safely apply the style of the most specific known class from the class list. These non-standard token types usually aren't likely to be handled by styles. We could also maintain a list of unexpected token types, but that does not seem necessary.