I added the optional parameter to cgitb.html() function and removed the
hardcoded css from the html code.

I think there are still several improvements to do:
- add documentation for the new parameter of cgitb.html()
- add the same parameter to cgitb.enable() (?)
- remove the cgitb.grey() function because it's not used anymore (?)
- perhaps minor improvements of html and css code

What do you think about it?