feat: support json/toml front matter

[slathrop]

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

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

[yyx990803]

Would be open to a PR that switch to gray-matter.

[colinbate]

The current integration of gray-matter is a good start, but it doesn't support the more common TOML frontmatter delimiters of +++. Unfortunately, gray-matter doesn't implement this auto-detection, but preliminaries does. I'm not suggesting another switch, as it should be something that VuePress can implement itself. At least until it gets added upstream.

If I get a chance in the next couple of days, I should be able to make a PR.

[colinbate]

In case anyone is interested, I decided to try and get the custom delimiter functionality implemented upstream in gray-matter directly. Hopefully, it will be sorted out before long.

jonschlinkert/gray-matter#75

[ulivz]

Fixed by #164