Add some way to turn off macros in a subtree

[tabatkins]

"Opaque" elements (style/script/pre/xmp, and anything with a bs-opaque attribute) don't process any of the markdown or markdown-ish stuff insdie of them; they're just html. But macros still work in there, and specs depend on that.

I should add a way to turn off macros as well; it causes issues for some contexts, like including LaTeX source.