cucumber test runner

[comboy]

Cucumber test runner needs to be implemented. Theoretically cucumber support "--format json" format, but it seems to return json after whole test suite is run, which is not nice, because we cannot show live updates of how test run is going (which is present for all other currently implemented test runners). But it could work as first implementation (however personally I get "only generation of JSON objects or arrays allowed (JSON::GeneratorError)" on some example project)