Switch from Misaka to alternate Markdown implementation

[fluffy-critter]

Context

Misaka is abandoned, and has led to too much hacking on the Publ side anyway.

Here are some more up-to-date alternatives which might be better choices:

• Python-Markdown
• mistletoe
• Mistune

These engines are all much more modular and up-to-date, and allow local customizations in a less-hacky way than the Misaka shoehornings. Being pure Python means one less thing that require a C++ compiler on Windows builds.

In order to make this change, there's a lot of stuff that will have to happen:

• Ensure that the image spec parsing is close enough (in particular, supporting multiline expressions with spaces in them and the other "extensions" we use for image sets) — this is likely the hard part
• Same for link spec parsing
• Ensure that MathJax can still work, ideally with minimal change to the Markdown text
• Update the pygments integration (and the line-numbering markup Publ generates)
• Probably a bunch of other non-obvious stuff

[fluffy-critter]

Looks like Misaka has been working on moving off of Hoedown. Hopefully this will fix the Hoedown-specific issues without breaking the parts of the API I've been relying on.

[fluffy-critter]

misaka is now officially abandoned and the maintainer has suggested moving to mistletoe or Mistune.

[fluffy-critter]

I've been using Mistune on Camptown and it's quite nice. Its smartypants functionality is a bit different (especially in emdash handling) and I'd probably have to completely rethink the way footnotes work (but that's been a thing I've been wanting to do anyway), but on the plus side it also has a comprehensive extension thing so I could probably properly support the Publ markdown extensions rather than having to hack them all in.

I haven't tried Mistletoe yet mostly because its docs are kind of hand-wavey and it isn't clear how to extend Markdown from it, and I don't have the patience to actually dive into the code.