Add a cron style timer, as well as being able to give a time as a string to run at.

Method cron() allows you to give a cron style input i.e. '15,45 * * * *' will trigger the timer at every 15 and 45 min past the hour.

Also add at(), pass a time as a string to have the timer trigger at that time e.g. at("16:00") will run at 4 in the afternoon. recurring_at() does the same but will trigger every 24 hours.

Any feedback is very welcome.