Added 'extra' field for extra data

Added an optional field `extra` that we store extra data in...

I also improved the documentation of `tags`, hopefully it's a little bit more obvious what it does.
Test works locally, there is nothing breaking except tasks already submitted can't be loaded anymore (well, maybe it works).

But new tasks will work regardless of whether or not they specify the `extra` property.
**Note,** the difference between task definition in `create-task-request.yml` and `task.yml`, the latter is used when returning tasks.

If anybody has a better name I'm open for suggestions, but I would like to deploy this soon. Shouldn't take any time.