feat(plugins): add toml support for frontmatter (#418)

* feat(plugins): add toml support for frontmatter

Currently frontmatter is expected to be yaml, with delimiter set to
"---". This might not always be the case, for example ox-hugo(a hugo
exporter for org-mode files) exports in toml format with the delimiter
set to "+++" by default.

With this change, the users will be able use frontmatter plugin to
support this toml frontmatter format.

Example usage: `Plugin.FrontMatter({delims: "+++", language: 'toml'})`

- [0] https://ox-hugo.scripter.co/doc/org-meta-data-to-hugo-front-matter/

* fixup! feat(plugins): add toml support for frontmatter

package.json

@@ -79,6 +79,7 @@
     "serve-handler": "^6.1.5",
     "source-map-support": "^0.5.21",
     "to-vfile": "^7.2.4",
+    "toml": "^3.0.0",
     "unified": "^10.1.2",
     "unist-util-visit": "^4.1.2",
     "vfile": "^5.3.7",