* Format JSON files.
Currently we store JSON as a single line in files. We should prettify it
like we do the other formats.
* Add output parsers to the frontmatter list.
* Cleanup frontmatter format parser options.
* Support writing frontmatter in TOML and JSON.
Right now we can read TOML or JSON frontmatter by inferring,
but we can only write frontmatter in YAML. This change allows the
frontmatter format to be explicitly set for reading and writing.
* Fix frontmatter formatter.
* Update Frontmatter formatter tests.
* Update frontmatter format docs.
* 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.
