Remove automatic lift:surround with="default" insertion

Here is a first try at this (which may need some more Markdown-related work).

Remove automatic lift:surround with="default" insertion, but wrap top level templates in <html></html> if the template contains multiple top level nodes and does not already contain <html>. This is necessary to support Markdown templates.

Fixes #1599 
Can we make this a named parameter? That'll make it clearer what the boolean is meant to compute, as I don't think it's really clear here.