Migration to v4
===============

v4 has the same external API as v3, but significantly changed internals. Plugin
authors should update their packages.

## Tokens and renderer

- [Tokens](https://github.com/markdown-it/markdown-it/blob/master/lib/token.js)
  are now classes, and allow arbitrary attributes.
  - new tokens are created with `token = state.push(type, tag, nesting)`.
    See [this commit](https://github.com/markdown-it/markdown-it/commit/4aabd5592ea55fb43d6a215b316c89c6f6f1f7db) to understand
    how to create tokens in new way.
- [Renderer](https://github.com/markdown-it/markdown-it/blob/master/lib/renderer.js)
  methods were unified. Number of custom renderer rules were significantly reduced.

## Other changes

- `.validateLink()` moved to root (MarkdownIt) class.