Add way to omit old events

[Jojodicus]

Running calendars for multiple years, they naturally fill up with events over time.
This can fill up the calendar with now-unneeded events.
As a secondary issue, this drives up bandwidth for each pull, though probably a minor issue in today's day and age.
It would be nice to filter/omit events older than a configurable time frame.

merged.ics:
  title: Awesome Calendar
  omit_older: 90d # new field, naming and format are still WIP
  urls:
    - ...

The default configuration should keep being to show all events.

[sdenike]

In hand with this one I have seen there are ways to remove attachments and reminders/alarms in events with some calendars so perhaps a way t further trim items in general from imported caledars