Revert the unescaping behavior on comments

Hello,

In #272 we introduced unescaping on HTML comments but at that time, we didn't really care about the conformance suite. I think that we should simply revert this behaviour ; maybe provide it through an option for the HTML render but I'm afraid that the number of options grow over the time if we wan't to allow customization of such things.

I would love to have your thoughts @mattr-. :smile: 

Thanks, have a nice day.