See commit for full details, includes:
- Introduce Footnotes extension
- Improve storage and use of source lines
- Improve recording of token position within source text
- Add logging of warnings for duplicate link/footnote definitions and unclosed front matter.
- Allow parsing of global context to renderers
- Disallow mixed backticks/tildes in code fence match (fixes miyuchina/mistletoe91)
Note this breaks from the mistletoe fork