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.

1993/plummer/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```sh
+```<!---sh-->
     make alt
 ```
 
@@ -12,21 +12,21 @@ code](#original-code) below.
 
 If you wish to speed the entry up, say to 500 microseconds between writes, try:
 
-```sh
+```<!---sh-->
     make clobber SLEEP=500 alt
 ```
 
 Alternatively if you wish to slow it down, say to 1500 microseconds between
 writes, try:
 
-```sh
+```<!---sh-->
     make clobber SLEEP=1500 alt
 ```
 
 
 ## To use:
 
-```sh
+```<!---sh-->
     ./plummer.alt number arg
 ```
 
@@ -42,7 +42,7 @@ bypass](https://hitchhikers.fandom.com/wiki/Bypass).
 
 ## Try:
 
-```sh
+```<!---sh-->
     ./try.alt.sh
 
     ./try.alt.sh 21701
@@ -70,7 +70,7 @@ course).
 
 ### Original build:
 
-```sh
+```<!---sh-->
     make all
 ```
 
@@ -82,7 +82,7 @@ Use `plummer` as you would `plummer.alt` above.
 
 ### Original try:
 
-```sh
+```<!---sh-->
     ./try.sh
 
     ./try.sh 21701