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/burton/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```sh
+```<!---sh-->
     make
 ```
 
@@ -18,7 +18,7 @@ For more detailed information see [2019 burton bugs](../../bugs.html#2019_burton
 
 ## To use:
 
-```sh
+```<!---sh-->
     ./prog < file
 
     ./cow file
@@ -27,7 +27,7 @@ For more detailed information see [2019 burton bugs](../../bugs.html#2019_burton
 
 ## Try:
 
-```sh
+```<!---sh-->
     ./try.sh
 ```
 
@@ -51,7 +51,7 @@ The output should look familiar.
 
 Compiles cleanly with:
 
-```sh
+```<!---sh-->
     WARN="-Wno-implicit-int -Wno-missing-variable-declarations -Wno-parentheses"
     cc -Wall -Wextra -Weverything -pedantic $WARN -include stdio.h -o prog prog.c
 ```
@@ -63,7 +63,7 @@ programs, it is silent upon success; any output demonstrates an error.  Instead
 of checking correctness, the [check.sh](check.sh) script can simply output the
 results:   (The Makefile contains the spoiler):
 
-```sh
+```<!---sh-->
     out=cat ./check.sh ./prog
 ```
 
@@ -97,7 +97,7 @@ interface.
 
 Included is a version without compilation warnings:
 
-```sh
+```<!---sh-->
     cc -Wall -Wextra -Weverything -pedantic -include stdio.h -o prog.clean prog.clean.c
 ```