Expand all leading tabs in markdown files

Given how problematic leading tabs in markdown files can be,
we have expanded all leading tabs in markdown files by:

```sh
find * -type f -name '*.md' -print |
  while read -r file; do
    rm -f "$file.tmp" ;
    expand < "$file" > "$file.tmp" ;
    mv -f -v "$file.tmp" "$file" ;
  done
```

Given the previous changes to `bin/pandoc-wrapper.sh` and other recent
commits and recent pull requests, the were were **NO** changes to
any HTML files.  This is a **good thing** as the expression goes.

No there are no leading tabs in any markdown files.
A check to catch any re-introduction of leading tabs
into any markdown file is on the way.

BTW: We do not have anything against leading tabs in files,
even though we have had to deal with their impact on forming
HTML from markdown files (as well as the calc source code).
It is just that this is one of those cases where tabs conflict
(sometimes caused by people wanting to change the classic
8-space tab model) with a desired outcome.

Perform `make www` to perform the above and test under macOS.

2015/mills1/README.md

@@ -45,35 +45,35 @@ From the Makefile:
     # Game parameters
     #
     DU=99999        # Time between steps in microseconds.
-		    # Make it larger if the game is too fast for you.
+                    # Make it larger if the game is too fast for you.
 
     DW=28           # Pipe-to-pipe spacing, in columns.
-		    # Farther apart is easier.
+                    # Farther apart is easier.
 
     DD=20           # Space between pipes.
-		    # The difference between DD and DW is the width of a pipe.
-		    # Narrow pipes are easier to fly though.
+                    # The difference between DD and DW is the width of a pipe.
+                    # Narrow pipes are easier to fly though.
 
     DH=8            # Height of gap in pipe.
-		    # Make this larger to make the game easier.
+                    # Make this larger to make the game easier.
 
     DA=100          # Acceleration due to gravity (scaled by 256).
-		    # Smaller values make you glide slower.
+                    # Smaller values make you glide slower.
 
     DI=-200         # Impulse velocity of a flap (scaled by 256).
-		    # Make larger if you want a less intense tapping experience!
+                    # Make larger if you want a less intense tapping experience!
 
     DB='"<o^="'     # Sprite for player when gliding.
-		    # HINT: Try using various emojis.
-		    # NOTE: The string MUST be enclosed inside double quotes.
+                    # HINT: Try using various emojis.
+                    # NOTE: The string MUST be enclosed inside double quotes.
 
     DF='"<ov="'     # Sprite for player when flapping.
-		    # HINT: Try using various emojis.
-		    # NOTE: The string MUST be enclosed inside double quotes.
+                    # HINT: Try using various emojis.
+                    # NOTE: The string MUST be enclosed inside double quotes.
 
     DG='"Tap to Flap!"'     # Instruction text.
-			    # Change to your native tongue.
-		    # NOTE: The string MUST be enclosed inside double quotes.
+                            # Change to your native tongue.
+                    # NOTE: The string MUST be enclosed inside double quotes.
 ```
 
 WARNING: if you have good eye-hand coordination and too much time on your hands