feat: support json front matter

[slathrop]

Similar to other SSGs (Hugo, etc.), add support in VuePress for JSON front matter (in addition to YAML and TOML [#141]).

• Select or write package equivalent to yaml-front-matter
  • Consider gray-matter
• Add reference to /lib/prepare.js
• Complete Implementation
• Complete Testing

[yeedle]

Objects seem to work, am I missing something? The following works for me

---
...
foo: { bar: "baz" }
...
---
{{ $page.frontmatter.foo.bar }}

resolves to "baz" for me.

[slathrop]

@yeedle Thanks. Yes indeedy YAML syntax is mostly a superset of JSON. But, for example, if you remove the space after a colon (e.g., bar:"baz") YAML parsers generally won't like that. More info here.

JSON support in front matter also means that you'd see opening and closing curly braces { } instead of --- for YAML (or +++ for TOML).

It's just a matter of "playing nice" and working with JSON without friction so-to-speak for those coming from other SSGs that directly support JSON, YAML, and TOML.

[yyx990803]

Closing and tracking together in #141