This enables the admonitions support from pulldown-cmark. This includes
a config option in case it causes problems with existing books.
I would like to make this extensible in the future, though I'm not sure
what that would look like. There's also some concerns with how this will
affect translations like mdbook-i18n-helpers, which we may need to work
out in a different way.
Closes https://github.com/rust-lang/mdBook/issues/2771
