Default in JSON should show only exercized rules

[cconcolato]

By default, only exercized rules (successful, error and warning) should be part of the report.
When the option parser is implemented (#48), we could have a option to show non-exercized rules.

[cconcolato]

Maybe the JSON should rather have a unchecked section.

[cconcolato]

The unchecked section should have all the rules from the corresponding spec, even if the format did not trigger them.

[rbouqueau]

Added unchecked section.

[rbouqueau]

@cconcolato @podborski Please close when QA-ed.

[rbouqueau]

I'm still interested in QA, thanks!