Add a `dispose()` method to `Docsify` to clean up the instance

[trusktr]

Feature request

Add a dispose() method so that Docsify can be cleaned up when no longer needed.

What problem does this feature solve?

This is needed for embedding Docsify (#799) on sites that switch to and from a view that has a Docsify instance (maybe they have multiple Docsify instances on different pages).

People currently can not clean up the instance when they will no longer render it.

What does the proposed API look like?

// later
window.$docsify.dispose() // or similar

and for people who will use the Docsify class directly,

const docsify = new Docsify(...)

// later
docsify.dispose()

How should this be implemented in your opinion?

Add a dispose method that cleans up route change listeners, anchor link click handlers, etc.