Add debut option to (color-less?) rich rendering.

[Carreau]

When rendering without colors it is sometime hard to compare text which appearance only changed based on color. For example, text and links look the same.

I think it would be nice to have a "debug" mode, that wrap some specific nodes in text markers,
for example [strong]...[/strong], [link]...[/link], this way we can actually debug or compare changes in rendering.