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.

2020/yang/README.md

@@ -1,20 +1,20 @@
 ## To build:
 
-```sh
+```<!---<!---sh-->-->
     make
 ```
 
 
 ## To use:
 
-```sh
+```<!---<!---sh-->-->
     ./prog [PIN] < input.txt > output.c
 ```
 
 
 ## Try:
 
-```sh
+```<!---<!---sh-->-->
     ./try.sh
 ```
 
@@ -22,7 +22,7 @@
 The author provided the file [cppp.cc](cppp.cc) which can preprocess the
 generated output to make them more acceptable to typical compilers. To build:
 
-```sh
+```<!---<!---sh-->-->
     make cppp
 ```
 
@@ -44,7 +44,7 @@ spoiler](spoiler.html) that shows how this entry was written.
 
 ### Synopsis
 
-```sh
+```<!---<!---sh-->-->
     echo "Hello world" | ./prog 12345 > encoded.c
 
     gcc -DPIN=12345 encoded.c -o ./decoded && ./decoded
@@ -63,14 +63,14 @@ Mile is a C code generator.  Run without arguments for the help message below:
 When run with an encoding PIN (first command line argument), Mile will
 read bytes from `stdin` and generate C code on `stdout`.  Example:
 
-```sh
+```<!---<!---sh-->-->
     echo "Hello, world!" | ./prog 56789 > encoded.c
 ```
 
 This generated code will reproduce the same input bytes on `stdout` when
 executed, if it was compiled with the same encoding PIN.
 
-```sh
+```<!---<!---sh-->-->
     gcc -DPIN=56789 encoded.c -o decoded
 ```