getTimeLeft () {
    if (this.taskCounter === 0) return '?'
    const averageTimePerTask = (Date.now() - this.beginTime.getTime()) / this.taskCounter
    const tasksLeft = this.totalTasks - this.taskCounter
    const millisecondsLeft = averageTimePerTask * tasksLeft
    return this.formatMilliseconds(millisecondsLeft)
  }
calculates the time left and outputs it in human readable format
calculation is based on the average time per task so far

@returns {string}