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.

1995/cdua/README.md

@@ -4,7 +4,7 @@ We recommend that you try the alternate version first so you can see what this
 entry looked like back in 1995 (with a configurable speed).  See the [original
 code](#original-code) section below for the original entry.
 
-```sh
+```<!---sh-->
     make alt
 ```
 
@@ -28,14 +28,14 @@ For more detailed information see [1995 cdua bugs](../../bugs.html#1995_cdua).
 
 ## To use:
 
-```sh
+```<!---sh-->
     ./cdua.alt
 ```
 
 
 ## Try:
 
-```sh
+```<!---sh-->
     make SLEEP=1500 clobber alt
 
     ./cdua.alt # press enter and watch it go much faster!
@@ -43,7 +43,7 @@ For more detailed information see [1995 cdua bugs](../../bugs.html#1995_cdua).
 
 You might also like to try:
 
-```sh
+```<!---sh-->
     make SLEEP=65000 clobber alt
 
     ./cdua.alt # press enter and watch it go much slower!
@@ -66,14 +66,14 @@ fast your computer is :-) or use both to see the difference, you can do so.
 
 ### Original build:
 
-```sh
+```<!---sh-->
     make all
 ```
 
 
 ### Original use:
 
-```sh
+```<!---sh-->
     ./cdua
 ```