Don’t mess with hl()+contents

What does hl() mean? It looks like “highlight an empty string”, but is actually 
“reset highlight style”. In addition if you are writing “hl()” function for new 
renderer you need to care about “contents” variable (in two places!: None for 
super() call and (contents or u'') after super() call) though function is 
actually doing nothing with it. It is just inconvenient.