feature: support of all markdown features and plugins in the sidebar.

## Feature request

It'd be great for all markdown features of pages to also work in the sidebar too. For example, suppose we want to use https://github.com/jhildenbiddle/docsify-tabs to put tabs in the sidebar, or something.

#### What problem does this feature solve?

Makes the site more customizable.

#### What does the proposed API look like?

From an end user perspective, they just write markdown, and it would work like it does in a page.

#### How should this be implemented in your opinion?

Not sure yet.

#### Are you willing to work on this yourself?

Yes, when I can find the time.