Ava: Produce JSON reports

[tymfear]

Please provide details about:

• What you're trying to do
  I want tova ability to have reports in JSON format to process it later to any other format I need.
• Why you can't use AVA for this
  Only TAP report is available at the time

Discussed with @novemberborn in #2583

[novemberborn]

Internally AVA has quite a lot of data about the test execution that is not easily represented in TAP form. I think it'd be more powerful to convert that into some other machine-readable format.