Allow hiding subject

[AffectivaNickB]

I'd like to use gcalcli to show conference room schedules on a screen, but I'd like it to only show who owns/created the event. Expected usage would be something like this:

gcalcli agenda --calendar 'Mad Damon' --details email --nodetails subject

[lasers]

I suggested to make --format for conky purpose in other issue, but it could work here too... Make gcalcli agenda --format=0.email,1.email,2.email to display first 3 event emails separated by comma.

One could specify gcalcli agenda --format=0.email to display only the latest event's email. I'm not sure we need agenda part anymore as it could be just gcalcli --format 0.email instead.