Allow backtick-quoting of <pre> tags.

The backwards-compatibility fix for code like:

    Here's the code example:<pre><code>
    alert("Blah");
    </code></pre>

was a bit too lose.  So tightened it up to only apply in case
where the <pre> or <pre><code> is at the end of line.

Fixes: #380
Merge branch 'master' into doc-tags