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.

2018/endoh1/README.md

@@ -1,13 +1,13 @@
 ## To build:
 
-```sh
+```<!---sh-->
     make
 ```
 
 
 ## To use:
 
-```sh
+```<!---sh-->
     ./prog < textfile > output.gif
 ```
 
@@ -16,7 +16,7 @@
 
 To see the input files and feed them to the program, redirecting to GIF files:
 
-```sh
+```<!---sh-->
     ./try.sh
 ```
 
@@ -40,7 +40,7 @@ least 2MiB of memory to run in.
 
 This program generates an animated GIF from a plain text.  Run:
 
-```sh
+```<!---sh-->
     cc -o prog prog.c
     ./prog < invisible.txt > invisible.gif
 ```
@@ -51,14 +51,14 @@ letters that leave the mark and ones that do not?
 
 Other examples:
 
-```sh
+```<!---sh-->
     ./prog < golem.txt > golem.gif
     ./prog < smily.txt > smily.gif
 ```
 
 The program itself has a hidden message.
 
-```sh
+```<!---sh-->
     ./prog < prog.c > prog.gif
 ```