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/duble/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```sh
+```<!---sh-->
     make
 ```
 
@@ -21,20 +21,20 @@ For more detailed information see [2019 duble bugs](../../bugs.html#2019_duble).
 
 ## To use:
 
-```sh
+```<!---sh-->
     ./prog file
 ```
 
 
 ## Try:
 
-```sh
+```<!---sh-->
     ./prog /tmp/drawing
 ```
 
 Open another window / terminal.
 
-```sh
+```<!---sh-->
     ./prog /tmp/drawing
 ```
 
@@ -42,13 +42,13 @@ Open more terminals and repeat...
 
 You might also wish to try a full screen size. For this try:
 
-```sh
+```<!---sh-->
     make clobber fullscreen
 ```
 
 Alternatively, if you want to change the size, do something like:
 
-```sh
+```<!---sh-->
     make clobber LINES=20 COLUMNS=20 all
 ```
 
@@ -69,7 +69,7 @@ the editor.
 
 After editing the file and quitting, try:
 
-```sh
+```<!---sh-->
     /tmp/drawing
 ```
 
@@ -97,7 +97,7 @@ alternate code might not work as well in macOS.
 
 ### Alternate build:
 
-```sh
+```<!---sh-->
     make alt
 ```
 
@@ -132,7 +132,7 @@ To build, type `make` (assuming gcc) or `make CC=clang`.
 
 Then you can start the program. It expects a file path as its first argument:
 
-```sh
+```<!---sh-->
     ./prog /tmp/drawing
 ```
 
@@ -216,7 +216,7 @@ Note: the drawing files `prog` generates are probably even more portable than
 The default size is the one of my xterm (see top of Makefile).
 You may set it to the full size of your terminal window by typing:
 
-```sh
+```<!---sh-->
     make clobber fullscreen
 ```