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