Cronjob is unusable without flask cli

It would be really helpful if I can register cronjobs without the need to execute `flask cronjob add`.

This step prevents me from easily incorporating your seemingly easy to use package to my project :-(

May I ask you why did you add this step? Why isn't decorating function with `cron` decorator and then initializing the crontab with current app enough? It would be sooo much easier to use this way.