add hack-a-round for language specified markdown code blocks

When language is specified in a markdown code block such as:

````
```c
    printf("Hello, world!\n");
```
````

is used, the resulting HTML as produced by `pandoc(1)` combined
with our revised `ioccc.css` is not intent the code block well.

A temporary "_hack-a-round_" is to turn the "markdown code language
specifier" into an HTML comment.  For example:

````
```<!---c-->
    printf("Hello, world!\n");
```
````

This commits turns off existing markdown code language specifiers
for now, while preserving our ability to restore them in the future
when a better  "_hack-a-round_" to the "markdown code language
specifier" problem is found.

The HTML impact if this "_hack-a-round_" is to produce HTML that
looks like this:

```
<pre class="&lt;!---c--&gt;"><code> .. </code></pre>
```

We might be able to create a complex set of classes for each markdown
code language (there won't be very many) in `ioccc.css`, or we might
find a more general CSS (or maybe `pandoc(1)`?) solution.

For now, this hack-a-round will have to do.

1991/index.html

@@ -155,8 +155,8 @@ a winning entry for the past 5 years!</p>
 would deny awards to more deserving entries. These categories have not
 been eliminated, and will likely be considered in future years.</p>
 <p>A number of entries made use of the following C pre-processor feature:</p>
-<div class="sourceCode" id="cb1"><pre class="sourceCode c"><code class="sourceCode c"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a>    <span class="pp">#define X define</span></span>
-<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>    <span class="pp">#</span><span class="er">X foo bar</span></span></code></pre></div>
+<pre class="&lt;!---c--&gt;"><code>    #define X define
+    #X foo bar</code></pre>
 <p>This is not permitted under ANSI C. Some ANSI C compilers do allow this. In
 the future, we do not want programs to use this feature. Because this request
 was not stated in the <a href="rules.txt">1991 rules</a>, we did not reject entries of this