Use child combinator (>) instead of descendant selector for
blockquote-tag-title color rules. Without this, a nested admonition
inherits the title color of its parent admonition instead of using
its own type's color.
Added nested admonition test case (important > note) to the existing
admonition test suite.
Fixes#3032
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
