Support writing frontmatter in multiple formats (#933)

* 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.

src/backends/backend.js

@@ -173,7 +173,7 @@ class Backend {
     return (entry) => {
       const format = resolveFormat(collectionOrEntity, entry);
       if (entry && entry.raw !== undefined) {
-        const data = (format && attempt(format.fromFile.bind(null, entry.raw))) || {};
+        const data = (format && attempt(format.fromFile.bind(format, entry.raw))) || {};
         if (isError(data)) console.error(data);
         return Object.assign(entry, { data: isError(data) ? {} : data });
       }