filters: apply HTML escaping

http://www.w3.org/International/questions/qa-escapes#use

filters/html-converters/txt2html

@@ -1,4 +1,4 @@
 #!/bin/sh
 echo "<pre>"
-cat
+sed "s|&|\\&amp;|g;s|'|\\&apos;|g;s|\"|\\&quot;|g;s|<|\\&lt;|g;s|>|\\&gt;|g"
 echo "</pre>"