Allow hooks to work for Sidenav

## Feature request

Allow hooks to work for Sidenav (and perhaps Navbar)

#### What problem does this feature solve?

I have a need where sidebar data is being generated (outside of my control) in a format different from that specified for './_sidebar.md'. With hooks enabled I can modify the sidebar information to the format Docsify wants.

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

No changes to the API

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

Add `beforeEach` and `AfterEach` hooks to `proto._renderSidebar` when invoking `proto.renderTo` in the same pattern as `proto._renderMain` in 'src/render/index.js'. Though I am not sure since I cannot claim to fully understand the code.

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

Only if a maintainer will discuss proposed changes in code and verify my changes! Last thing I want to do is upload a poor PR that breaks things!!!