JSON for plugin definitions

[borekb]

For people who don't like writing YAML (

No:
  Body:
    Wants:
      To:
        Write:
          - yaml

https://twitter.com/rothgar/status/938790206140010498

), we could support JSON as well. Actually, JSON with comments that VSCode uses would be even better – it strikes the right balance between being close to JSON and supporting some essential features for human-writable config files.

We'd need to support both YAML and JSON with comments, at least in the short term, but that shouldn't be a big deal, everything parses into the same internal data structures.

[borekb]

Two quick notes:

• Prettier now supports YAML.
• I'm becoming a fan of TOML. See also INI file format vs. others (JSON, YAML, XML...) #914 (comment).