Email 2.0: enable to disable the preheader feature

The new pre-header feature is quite easy to use, but does not support the dynamic content.

The workaround for this is to create a preheader section in the email. The drawback of this is that, ni this case, the user may enter the pre-header text in 2 places and make errors.

So it would be great is the preheader feature could be de-activated either for the whole instance or, better yet, at template level (for instance using a specific mkto Meta declaration).

-Greg