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.

2019/karns/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```sh
+```<!---sh-->
     make
 ```
 
@@ -21,13 +21,13 @@ For more detailed information see [2019 karns bugs](../../bugs.html#2019_karns).
 
 ## To use:
 
-```sh
+```<!---sh-->
     ./prog < textfile_that_fits_on_the_screen
 ```
 
 You might want to type:
 
-```sh
+```<!---sh-->
     reset
 ```
 
@@ -36,13 +36,13 @@ after running the program to restore the terminal to its previous state.
 
 ## Try:
 
-```sh
+```<!---sh-->
     ./try.sh
 ```
 
 If you have more time you might want to try:
 
-```sh
+```<!---sh-->
     ./prog < README.md
 ```
 
@@ -82,7 +82,7 @@ connected with anything.
 
 This program usually compiles under both GCC and clang. Build with:
 
-```sh
+```<!---sh-->
     $(CC) -std=c99 -o tbfs prog.c
 ```
 
@@ -90,13 +90,13 @@ where `$(CC)` is cc, gcc, clang, or some other C compiler.
 
 You can then run it with:
 
-```sh
+```<!---sh-->
     cat <input file> | ./tbfs
 ```
 
 Examples:
 
-```sh
+```<!---sh-->
     ./tbfs < maze
     ./tbfs < prog.c
 ```