4.0.0

This update adds support for transforming literal URLs and autolinks
to be transformed to references when they point to issues, pull
requests, commits, issue comments, pull request comments, or commit
comments. For example,

<https://github.com/wooorm/remark/commit/1234#commitcomment-5678>

Is transformed to:

[wooorm/remark@`1234` (comment)](wooorm/remark@1f2a#commitcomment-5678)

Images, and links with text other than their respective URL are left
untouched.