`gcalcli agendaupdate` column for `action`

[michaelmhoffman]

The input for gcalcli agendaupdate (#550) could be enhanced by an action column which specifies what method is executed on the event in the row. I envision the following potential actions:

• [x] patch (the default, currently in process of implementation)
• [x] ignore: do nothing. This would allow having a whole sheet of events in the input for context but changing only a few
• [x] delete
• [x] insert: create new event (but patch with a blank id field should also insert a new event)
• [ ] update: like patch, but replaces the whole event instead of using patch semantics

delete is a much higher priority than update. I'm not including move because a better interface for that is to have a calendar column and just to specify the new calendar in that column.