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.

2000/thadgavin/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```sh
+```<!---sh-->
     make alt
 ```
 
@@ -16,7 +16,7 @@ looked like back in 2000 (with a configurable speed). See the
 
 ## To use:
 
-```sh
+```<!---sh-->
     ./thadgavin.alt
 ```
 
@@ -27,7 +27,7 @@ You can press 'q' at any time to exit.
 
 If you have SDL1 installed:
 
-```sh
+```<!---sh-->
     ./thadgavin_sdl
 ```
 
@@ -43,13 +43,13 @@ microseconds) for the SDL version.
 
 To change the SDL one to something like `15000` you can do:
 
-```sh
+```<!---sh-->
     make SDL_SLEEP=15000 clobber alt
 ```
 
 and to change the curses one to `40`:
 
-```sh
+```<!---sh-->
     make SLEEP=40 clobber alt
 ```
 
@@ -75,14 +75,14 @@ is not a problem there.
 
 ### Original build:
 
-```sh
+```<!---sh-->
     make all
 ```
 
 
 ### Original use:
 
-```sh
+```<!---sh-->
     ./thadgavin
 ```
 
@@ -91,7 +91,7 @@ is not a problem there.
 
 If you have SDL1 installed:
 
-```sh
+```<!---sh-->
     ./thadgavin_sdl
 ```
 
@@ -110,14 +110,14 @@ less than dazzling. We could not test this entry in DOS mode.
 
 Compile using DJGPP as follows:
 
-```sh
+```<!---sh-->
     gcc thadgavin.c -o thadgavin.exe -Wall -lm -O6 -mpentium -fomit-frame-pointer -ffast-math
 ```
 
 
 ### To use under Windows, X-Windows or macOS using the Simple DirectMedia Layer:
 
-```sh
+```<!---sh-->
     gcc -O6 -lpthread -g -o thadgavin thadgavin.c -lSDL -DSDL -lm
 ```
 
@@ -125,7 +125,7 @@ Compile using DJGPP as follows:
 ### To use curses:
 
 
-```sh
+```<!---sh-->
     gcc -O6 -lpthread -g -o thadgavin thadgavin.c -lncurses -lm
 ```