Add ability for cron repeatable job with startDate

Created by: nirpeer

In this following example, the repeatable job will start run at 02.02.2020, 12:00, for every 2 minutes.

queue
      .add(
        'repeat',
        { foo: 'bar' },
        {
          repeat: {
            cron: '*/2 * * * * *', // every 2 seconds
            startDate: new Date('2020-02-02 12:00:00')
          }
        }
      )

This solution handles https://github.com/OptimalBits/bull/issues/764 issue.