[Cron Jobs] Add ability to run periodic tasks within proxy.py core

[abhinavsingh]

Is your feature request related to a problem? Please describe.
See https://github.com/abhinavsingh/proxy.py/blob/develop/proxy/proxy.py#L81-L83 for background. Currently, core performs constant sleep while server is running. This is a good place to introduce cron style feature.

Describe the solution you'd like
Ability to write cron jobs within plugins. Example, to periodically fetch ad-blocker rule list every hour or so.