Support RFC 7953 Availability

Should read up on https://www.rfc-editor.org/rfc/rfc7953.html and consider if/how this needs to be supported.

See also:

* https://github.com/niccokunzmann/python-recurring-ical-events/issues/143
* https://github.com/collective/icalendar/issues/654