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-lock.json

@@ -55,6 +55,7 @@
         "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",
@@ -5548,6 +5549,11 @@
         "url": "https://opencollective.com/unified"
       }
     },
+    "node_modules/toml": {
+      "version": "3.0.0",
+      "resolved": "https://registry.npmjs.org/toml/-/toml-3.0.0.tgz",
+      "integrity": "sha512-y/mWCZinnvxjTKYhJ+pYxwD0mRLVvOtdS2Awbgxln6iEnt4rk0yBxeSBHkGJcPucRiG0e55mwWp+g/05rsrd6w=="
+    },
     "node_modules/tough-cookie": {
       "version": "4.1.3",
       "resolved": "https://registry.npmjs.org/tough-cookie/-/tough-cookie-4.1.3.tgz",