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/giles/README.md

@@ -1,20 +1,20 @@
 ## To build:
 
-```sh
+```<!---sh-->
     make
 ```
 
 
 ## To use:
 
-```sh
+```<!---sh-->
     ./prog infile.wav outfile.wav
 ```
 
 
 ## Try:
 
-```sh
+```<!---sh-->
     ./try.sh
 ```
 
@@ -62,7 +62,7 @@ Interesting fact: The size of the program from `./iocccsize -i` is 2019.
 
 #### How to use:
 
-```sh
+```<!---sh-->
     ./prog input [output [size [dry [wet [damping]]]]]
     prog.exe input [output [size [dry [wet [damping]]]]]
 ```
@@ -87,7 +87,7 @@ The default settings are designed to make the effect obvious rather than subtle,
 so it may be a good idea to adjust the ratio of "dry" to "wet", to give a more
 pleasant result:
 
-```sh
+```<!---sh-->
     ./prog in.wav out.wav 1.8 80% 15%
 ```