Allow plugins to handle raw embedded markdown content

## Feature request

See title.

Related:

- #1499
- #2000

#### Problem or desire

Plugins are unable to process [embedded](https://docsify.js.org/#/embed-files?id=embed-files) markdown content because docsify does not make this content available via a plugin's `beforeEach` hook. Instead, docsify retrieves the embedded markdown content, transforms it to HTML, and makes it available for the first time to a plugin via the `afterEach` hook. This prevents plugins like [docsify-tabs](https://jhildenbiddle.github.io/docsify-tabs/) from working with embedded content.

#### Proposal

See title.

#### Implementation

TBD