Add custom endpoints for iCal / Google / Yahoo

[remcotolsma]

Add custom endpoints for iCal / Google / Yahoo https://codex.wordpress.org/Rewrite_API/add_rewrite_endpoint

https://www.example.com/events/test-event/icalendar https://www.example.com/events/test-event/google-calendar https://www.example.com/events/test-event/yahoo-calendar

http://carlsednaoui.github.io/add-to-calendar-buttons/generator/generator.html

https://www.google.com/calendar/render?action=TEMPLATE&text=Test&dates=20121212T111200Z/20121212T111200Z&details=&location=Tillewei&sprop=&sprop=name:

http://calendar.yahoo.com/?v=60&view=d&type=20&title=Test&st=20121212T121200Z&dur=0000&desc=&in_loc=Tillewei

data:text/calendar;charset=utf8,BEGIN:VCALENDAR%0AVERSION:2.0%0ABEGIN:VEVENT%0AURL:http://carlsednaoui.github.io/add-to-calendar-buttons/generator/generator.html%0ADTSTART:20121212T111200Z%0ADTEND:20121212T111200Z%0ASUMMARY:Test%0ADESCRIPTION:%0ALOCATION:Tillewei%0AEND:VEVENT%0AEND:VCALENDAR

• http://icalendar.org/
• https://en.wikipedia.org/wiki/ICalendar
• http://stackoverflow.com/questions/1310420/difference-between-icalendar-ics-and-the-vcalendar-vcs
• http://stackoverflow.com/questions/22757908/google-calendar-render-action-template-parameter-documentation
• http://stackoverflow.com/questions/20238235/link-for-sharing-an-event-to-yahoo-calendar

[remcotolsma]

Added in https://github.com/pronamic/wp-pronamic-events/commit/36619bfaccd503653a6b0e3817a6a37c5f577b19.