* Clean up frontmatter formatter.
* Move `formatToExtension`.
* Use plain objects for file formatters.
* Use same parsers for files and frontmatter.
We want to use our file parsers for frontmatter, instead of the builtin
ones, as they process some formats (images, dates) properly.
* Cleanup YAML frontmatter parser code.
Switches back to custom frontmatter stringification
until support lands in preliminaries. This is necessary
because we use custom schemas for certain data types,
such as dates and times.
