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.

1995/leo/README.md

@@ -1,6 +1,6 @@
 ## To build:
 
-```sh
+```<!---sh-->
     make all
 ```
 
@@ -18,7 +18,7 @@ For more detailed information see [1995 leo bugs](../../bugs.html#1995_leo).
 
 ## To use:
 
-```sh
+```<!---sh-->
     ./leo [ deep ] [ right ] [ Variable ] [ cycle [ freq ] ]
 ```
 
@@ -27,7 +27,7 @@ See the author's information below on these details.
 
 ## Try:
 
-```sh
+```<!---sh-->
     ./try.sh
 ```
 
@@ -52,7 +52,7 @@ characters on the command line - anything that's not a number is a candidate.)
 
 The usage is:
 
-```sh
+```<!---sh-->
     ./leo [ deep ] [ right ] [ Variable ] [ cycle [ freq ] ]
 ```
 
@@ -68,7 +68,7 @@ to be present if you want to enter `freq` (and you usually do).
 
 Some options to try (besides the obvious):
 
-```sh
+```<!---sh-->
     ./leo 1 | cat - /dev/tty | gs -
 ```
 
@@ -191,19 +191,19 @@ the opening for step 1 is not a single line but a cross:
 
 To watch it repeatedly (as a screen saver), use:
 
-```sh
+```<!---sh-->
     ./leo 2 | cat - /dev/tty | gs -
 ```
 
 The command line:
 
-```sh
+```<!---sh-->
     ./leo Variable 1 | cat - /dev/tty | gs -
 ```
 
 draws the same as above. Try:
 
-```sh
+```<!---sh-->
     ./leo Variable 1 1 | cat - /dev/tty | gs -
 ```
 
@@ -228,7 +228,7 @@ If `Variable` is used, the actual value of `cycle` doesn't matter,
 as far as it is > 1. To figure out what difference
 `cycle` makes when `Variable` is not used, try
 
-```sh
+```<!---sh-->
     ./leo Variable 37 80 | cat - /dev/tty | gs -
 ```