Add timestamp to JSON formatter scenario output

We're recording Selenium based tests and use this to compute time offsets to skip around in our screen recordings.

Currently there is no easy method to get an *absolute* time from Cucumber output. The only approach is to see when Cucumber starts/terminates and subtract `duration` properties from that time. This is skewed by various other things (for example browser starting up before the first step gets executed).

Summing up `duration` is quite a hassle, and can be easily circumvented by this patch. It may be of use for others, so I am requesting a pull here.