Add support for new iCalendar-RFC-7986 attributes

[muhammadyusuf-kurbonov]

Hello. Thanks for this lib :).

I want some attributes from the new RFC-7986.
Please implement them. Also, please add the ability to add custom non-X attributes (as a temporary workaround for using new attributes in the future).

[sebbo2002]

Hey, sorry for the late reply. What attributes are you talking about specifically? What are you missing? I would like to avoid adding generic attributes, as this can very quickly invalidate the file or, for example, result in duplicate attributes.

[muhammadyusuf-kurbonov]

Color, REFRESH-INTERVAL at most. Thanks. Don't you know if Google Calendar supports these attributes?

[sebbo2002]

Both Google Calendar and Apple Calendar ignore the color attribute. I do not assume that this has changed, as they only support RFC-5545. If this has changed in the meantime, I would be willing to add it accordingly. But if the attribute doesn't work in Google Calendar nor Apple Calendar, it probably doesn't make sense to implement it.

A quick google revealed that the refresh interval cannot be changed by the ical file for Google Calendar. Try it yourself again if you want to be sure. You can set REFRESH-INTERVAL with the ttl() method.