Client specific attribute X-GOOGLE-CONFERENCE

[mrjackdavis]

Hey,

Just wondering what the appetite is for client specific attributes such as X-GOOGLE-CONFERENCE which is used by Google Calendar to render the Join Meeting (it references the link in X-GOOGLE-CONFERENCE, location alone doesn't work)

If you're open to it, I'm happy to make the PR.

[Kotelnikovekb]

Hello, it would really be nice to add the ability to create your own non-standard tokens (for example X-TELEMOST-REQUIRED)