[feature] Add an option to specify cron-like rules for checking for diffs

Use case:
- I *really* want to be notified ASAP about new stuff on given website. So I set the low interval of 5 mins
- But I don't like the idea of wasting resources (both from my side and site owner) - so I'd like to skip the checks on some specific hours where I sleep

Using cron-like rules I could save some resources.

Please consider this feature request, thanks!

The issue names no files or tests; review closed pull request #231 first, then locate the existing check-interval scheduler. Done means users can configure cron-like rules that skip checks during selected hours while preserving checks at permitted times.

Written by the indexing model from the issue text.