rfc6868 support in parsing

[AThomsen]

About rfc6868 :

This specification updates the data formats for iCalendar ([RFC 5545](https://datatracker.ietf.org/doc/html/rfc5545))  and vCard ([RFC 6350](https://datatracker.ietf.org/doc/html/rfc6350)) to allow parameter values to include certain characters forbidden by the existing specifications.

As I now know the parsing code, I offer to implement this in the parser if it's of any interest.

[allenporter]

Yes, would be a pleasure to have more of your contributions. Just a few thoughts as you get started:

• This is definitely in the spirit of what we'd want to support. Simple extensions from other RFCs are welcome.
• I updated the implementation a small amount based on some style preferences, and combined it with the old "fast" approach which was removed since your approach supercedes it
• Please add a small section in the README about rfc5545 extensions and list this one. i can add a couple other small ones later. Appreciate your prompt here as I realize now there are a few i have missed in the past I am interested in supporting https://datatracker.ietf.org/doc/html/rfc7986#section-4 as well
• It would probably be useful to have a mix of unit tests e.g. test_property.py for the various corner cases in the RFC plus a few of the larger file based tests.