Add memo for processor to `MarkdownHooks`

Some plugins may perform heavy work in preparation to make the
transformer light-weight. A good example of this is
`rehype-starry-night`.

Without this change, an interactive markdown editor which includes
`rehype-starry-night` is very sluggish. With this change, performance is
as good as if `rehype-starry-night` isn’t even used.


Closes GH-909.

Reviewed-by: JounQin <admin@1stg.me>
Reviewed-by: Titus Wormer <tituswormer@gmail.com>