Additional fixes to the html.sed script

It now translates the characters '|' (pipe), '=' (equals) and '%'
(percent).

Checks were added in md2html.sh in the -s token=value so that token
cannot contain an = (equal), | (pipe) or a % (percent) and the value
cannot contain a | (pipe).

The reason that we cannot disallow a ';' or '&' is because the html
entities use those characters so if the sed script were to translate the
problematic characters it would cause a problem in the script.

Ran make www which updated 1988/robison/index.html and
1990/theorem/index.html.

1986/bright/README.md was fixed with the number of blank lines between
sections.

1990/theorem/index.html

@@ -406,7 +406,7 @@
   </a>
   <h1>The International Obfuscated C Code Contest</h1>
   <h2>1990/theorem - Best of Show</h2>
-  <h3>numerically solves the equation y'=f(x y)</h3>
+  <h3>numerically solves the equation y'&equals;f(x y)</h3>
 </div>
 
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