Implementation's registry

[niquola]

We can track implementations from the spec.
If we agree on the test results report, we can automatically build a
supported features matrix for different implementations.

For that, we can create a folder implementations/, where implementations can be pull-requested as a separate json file like:

{
  name: 'my fancy implementation,
  description: '.....',
  site: 'http://..',
  // index of test results
  tests_report: 'http://.../tests/index.json'
}

// tests/index.json
{ tests: [{name: ..., resultUrl: ...},{name: ..., resultsUrl: ...}]}