Implement xCal and xCard support

Useful links:
  * [RFC6321](https://tools.ietf.org/html/rfc6321), xCal: The XML Format for iCalendar,
  * [RFC6351](https://tools.ietf.org/html/rfc6351), xCard: vCard XML Representation.

:exclamation: **Not ready to merge**, this is a WIP :exclamation: