Python-comrak

* Add a Default derive and Ast::new to make ASTs programmatically
constructible. (101, Sunjay Varma and 102)

* Add a callback to fill in broken reference links, per pulldown_cmark's
Parser::new_with_broken_link_callback. (100, Sunjay Varma)
* Update to latest spec. (99)

* Fix a bug in anchor generation; it should now be on par with GitHub's. (97, Clifford T. Matthews)
* Expose anchor generation for use in library consumers. (94, Clifford T. Matthews)

* Invert default-false `safe` flag to default-false `unsafe_` flag. If you
were not enabling safe mode before, you'll need to enable unsafe mode now.

* Keep up-to-date with the spec.

* Significant test coverage and code clean up. (82, 83, Brian Anderson)
* Description list support. (86, Ayose Cazorla)
* Example use of comrak to convert CommonMark documents into S-expressions. (86, Ayose Cazorla)
* Footnotes are now enabled via an extension option, not a flag of its own. (87)
* Extend `cmark-gfm` compatibility to include all extension and regression tests. (87)