Support for custom (callable) intervals

Hi!
Just to give more control to the user, I think it would be nice to allow defining callable intervals.

E.g.
```
schedule.every(lambda: random.randint(5, 50)).seconds.do(job)
```

This would be more flexible than #154 .

Start at the schedule.every(...).seconds.do(job) entry point shown in the issue and trace how the interval is stored and used for scheduling. Confirm the callable form accepts values such as random.randint(5, 50) and that the resulting interval controls job execution as requested.

Written by the indexing model from the issue text.