persist and maintain arbitrary front matter on import

[cloverich]

#134 implements basic support for importing a Notion export which is nearly the same as #132, excepting a few notion specific things (like the ids in titles) and a potential lack of generalization. It is also the first attempt to import anything at all and is done in experimental fashion. One issue I ran into was some of my exported documents have arbitrary front matter, structured like this:

# Guilded

Link: <https://www.guilded.gg/jobs/full_stack_engineer>
Gist: Gaming comunity
Full stack role
I like the tech but consumer focused 
rank: 2
stage: precontact

...note content (or not) here

Base support for importing notion was to grab the documents, titles, note links, images, etc. But I don't have the concept of arbitrary front matter support yet, but also don't want to just drop this information, or dump it into the body without use. Instead, if its straight forward enough, it would be great to allow for arbitrary front matter; although Chronicles doesn't do anything with it yet (if it does, will be done here: #127) maintaining it would allow for future use and avoid wiping it.

• Parse front matter from the document
• Stuff it into a JSON column
• When document write is called, merge it into the generated front matter