Scedule Previous method implementation

[vincentmrg]

Hello,
I added the Previous method to the Schedule interface, as well as the implementation of this method to SpecSchedule. This method works on the same principle as the Next () method but on the other hand allows you to deduce the previous date where the schedule would have been activated.

This method is useful to us in order to define a list of dynamic configurations scheduled by cron and to deduce which configuration to use when applying these configurations.

[thinkerbot]

Interestingly enough I just went looking for a Previous method and was happy to see this PR. My use case is trying to identify a time interval defined by a specific time and a cron schedule.

[omaskery]

Hi, this PR looks perfect for my current problem. Is there anything I can do to help move this PR along? I've had a quick look at the changes and it all looks sane and tested :)

Edit: bad news @thinkerbot, @vincentmrg, it looks like a previous, almost identical PR was denied last year as being a use case outside the scope of this library - which is fair enough: https://github.com/robfig/cron/pull/216/files

I suspect there's probably room in the world for somebody to factor out the underlying cron schedule expression parsing and "next/prev" calculation, but this library (more focused on running jobs looking forward) probably isn't that place.

[vincentmrg]

@omaskery Yep, I saw that a little late. As I need it, I still opened this PR.
@robfig free to you to accept or not this PR, it seems that some people find it useful.