New output filter ‘debug’ for debugging purpose
Major and minor syntax classes, XML lexer enhanced

The xml lexer does not use encoding/xml anymore and thus 
doesn’t change the appearance of the xml
Use major and minor highlight classes in xml/html
Rename TokenMajor/TokenMinor to Type…
More go like using channels
Remove filter type, store functions

I think it’s cleaner to store Highlight/Render functions directly
instead of the unused filter types which contain the 
Highlight/Render methods.
Add in-between filters (sanitizers)