let the user decide to remove the html comments on html body emails

We use the email api as part of our CI suite and we *mark* the html body using comments to check if everything is correct. 
We don't think that is up to the framework to decide if html comments __should or not__ be stripped. That said, the framework should not alter the body other than the required manipulation to send the email.

I've added a new config parameter that let the use to decide if html comments should be removed