Dates should follow ISO 8601

[tobiasvl]

The date in game.json should comply with ISO 8601 (2017-12-07) instead of the American date format (12/07/2017).

See also gbdev/homebrewhub#6

[avivace]

Dates object in the json files here could be completely arbitrary, so I'll go with it.

We also expect in the near future (soon™) to have a simple and html form to easily add games, which converts the addition to a compliant PR here, so we could implement there a simple date picker, without having to force US or EU standards.