Add TypeScript support

As explained in this issue, the new node-cron version had a breaking change.

The third param, of schedule function, immediateStart was removed and replaced by a options.

The behavior of immediateStart was replaced by the option scheduled.

So, to fix this issue we had two options:

1 - Update the lib https://github.com/DefinitelyTyped/DefinitelyTyped/tree/master/types/node-cron

2 - Introduce a typings on node-cron project.

[jacob-hd]

Typings would be great.

[merencia]

because I don’t want to expose the scheduler.

For example, now I have something like this:

constructor(cronExpression, func, options) {
   //...
   let scheduler = new Scheduler(cronExpression, options.timezone, options.recoverMissedExecutions);
  //...

  this.start = () => {
      scheduler.start();
   };
}

If I change that to methods I need to put the scheduler in the instance, the scheduler may be accessed through scheduledTask.scheduler.

constructor(cronExpression, func, options) {
   //...
   this.scheduler = new Scheduler(cronExpression, options.timezone, options.recoverMissedExecutions);
  //...
}

start() {
  this.scheduler.start();
}