Add `fallback` prop to `MarkdownHooks`

The `<MarkdownHooks/>` component now supports a new prop named
`fallback`. This fallback is displayed while the initial content hasn’t
loaded yet. The name `fallback` was chosen to match the same prop from
`<Suspense/>`.

Closes GH-897.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>