feat(runner): make runner an event emitter and log passes or failures from the launcher

Now, for runs with multiple capabilities, the launcher will output '.' or 'F'
for each pass or fail instead of just '.' for every chunk of data.
TODO - complete the event emitter API for the Cucumber runner.
docs(faq): clarify that browser logs are always level WARN and higher
feat(launcher): launcher outputs a final summary of how the browsers did