Add human readable date in ISO 8601 format to all-events.json

[jonatan-ivanov]

Right now https://developers.events/all-cfps.json contains a somewhat human readable date field but but https://developers.events/all-events.json does not. I think both should.

[HanzCEO]

This sounds like a good idea, I like the duration formatting.
It would be very much appreciated for you to initiate a PR