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.

website/site/content/docs/configuration-options.md

@@ -95,9 +95,12 @@ You may also specify a custom `extension` not included in the list above, as lon
 - `yml` or `yaml`: parses and saves files as YAML-formatted data files; saves with `yml` extension by default
 - `toml`: parses and saves files as TOML-formatted data files; saves with `toml` extension by default
 - `json`: parses and saves files as JSON-formatted data files; saves with `json` extension by default
-- `frontmatter`: parses files and saves files with data frontmatter followed by an unparsed body text (edited using a `body` field); saves with `md` extension by default; default for collections that can't be inferred
+- `frontmatter`: parses files and saves files with data frontmatter followed by an unparsed body text (edited using a `body` field); saves with `md` extension by default; default for collections that can't be inferred. Collections with `frontmatter` format (either inferred or explicitly set) can parse files with frontmatter in YAML, TOML, or JSON format. However, they will be saved with YAML frontmatter.
+- `yaml-frontmatter`: same as the `frontmatter` format above, except frontmatter will be both parsed and saved only as YAML, followed by unparsed body text
+- `toml-frontmatter`: same as the `frontmatter` format above, except frontmatter will be both parsed and saved only as TOML, followed by unparsed body text
+- `json-frontmatter`: same as the `frontmatter` format above, except frontmatter will be both parsed and saved as JSON, followed by unparsed body text
 
-Collections with `frontmatter` format (either inferred or explicitly set) can parse files with frontmatter in YAML, TOML, or JSON format. On saving, however, they will currently be saved with YAML frontmatter. (Follow [Issue #563](https://github.com/netlify/netlify-cms/issues/563)) to see when this changes.)
+The explicit `yaml-frontmatter`, `toml-frontmatter`, and `json-frontmatter` formats above do not currently support custom delimiters. We use `---` for YAML, `+++` for TOML, and `{` `}` for JSON. If a file has frontmatter inside other delimiters it will be included as part of the body text.
 
 
 ### `slug`