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.