Support more interval definitions for repeatable jobs

Created by: cuchi

Hi!

We are currently migrating from agenda, but there is this little problem keeping us from doing that. The cron notation is a very good idea to schedule repeatable jobs, but there are cases it is impossible to use, like representing an interval definition of every 90 minutes.

To support this, RepeatOpts could be modified to accept other formats besides cron, which could be:

• How many seconds or millis, like in setInterval
• A string to be parsed by human-interval, which is used by agenda

I do not know the complexity of this enhancement, but it could be a good idea for the next feature release, what do you think?

Maybe related to #912 (closed)