ASHIQUE-KV

/**
 * Returns a {key: value} object where the key is a start date and the value is the date + 1 of the type of interval
 * to the start date. When for weeks or months, it shows just the first date of the week/month.
 *
 ** For days (start: '2017-12-25', end: '2018-01-02', interval: 'day'):
 { '2017-12-25': '2017-12-26',
  '2017-12-26': '2017-12-27',
  '2017-12-27': '2017-12-28',
  '2017-12-28': '2017-12-29',