This changes it so that header and `<dt>` tags manually written as HTML
are not modified (no anchor, no id, etc.). This is to avoid mangling any
HTML that the user explicitly crafted.
I'm not sure what the fallout from the headers might be, since I'm not
100% sure there aren't uses where the user wanted mdbook to modify
manual HTML. However, I don't see any in rust-lang's use.
This enables the definition lists support from pulldown-cmark.
This includes a config option in case it causes problems with existing
books.
Closes https://github.com/rust-lang/mdBook/issues/2770