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

## 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 (https://github.com/docsifyjs/docsify/issues/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?

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

and for people who will use the `Docsify` class directly,

```js
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.