code: function schedule(expression, func, options) {
          let task = createTask(expression, func, options);
          storage.save(task);
          return task;
      }
docs: Creates a new task to execute given function when the cron
  expression ticks.
  
  @param {string} expression - cron expression.
  @param {Function} func - task to be executed.
  @param {Object} options - a set of options for the scheduled task:
  - scheduled <boolean>: if a schaduled task is ready and running to be
  performed when the time mach with the cron excpression.
  - timezone <string>: the tiemzone to execute the tasks.
  
  Example:
  {
  "scheduled": true,
  "timezone": "America/Sao_Paulo"
  }
  
  @returns {ScheduledTask} update function.