feat: support for TOML front matter (#141)

[ghost]

I took a stab at swapping out yaml-front-matter for gray-matter to enable TOML front matter. In addition, JSON front matter also works (but I believe it did with yaml-front-matter as well).

Here's an example of how to use TOML front-matter:

---toml
title = 'This is a title'
[[meta]]
  name = 'description'
  content = 'hello'
---

I also refactored the front matter parsing code a bit, making use of the existing parseFrontmatter util.

P.S. This is my first pull request. Please let me know if I am missing anything 😄

[yyx990803]

Great job for a 1st PR!

[rwxrob]

Very nice indeed and not in the documentation that I could find. I will look again and maybe submit a documentation PR to cover it if needed.

I'm a little torn on the ---toml convention but it makes a lot of sense since it allows whatever possible language to be used or added later without inventing yet another marker (as Hugo does for YAML, TOML, and JSON).

[ulivz]

@robmuh Welcome to open a PR for that. Features added in the early days may have forgotten to add doc, but should be rare.