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.

1992/imc/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```sh
+```<!---sh-->
     make all
 ```
 
 
 ## To use:
 
-```sh
+```<!---sh-->
     # text mode
     ./imc -text
 
@@ -19,7 +19,7 @@
 
 To see a number of options and configurations:
 
-```sh
+```<!---sh-->
     ./try.sh
 ```
 
@@ -164,7 +164,7 @@ standard error to indicate its progress (one dot per pixel line).
 
 Here are a couple of good pictures to draw:
 
-```sh
+```<!---sh-->
     ./imc -s 512,512 -f 600000 -l 512 -m 16 -c.00805,.74274 > screen1.ras
     ./imc -s512,512 -j-.523,-.535 > screen2.ras
 ```