add option to not delete past events

[non-bin]

addresses #117 by adding a toggle, to disable deleting past events

[non-bin]

ok it all works now, but I'm not 100% sure it covers all edge cases. The documentation mentions start.date but I think that's just for recurring events which are ignored by this method anyway, but then for start.dateTime it says:

A time zone offset is required unless a time zone is explicitly specified in timeZone.

but doesn't mention whether a zone will always be returned in the dateTime by the API, it seems to always put a timezone in both though.
It's annoying that we can't just use the standard API, which would just return a Date object and remove all these problems.

https://developers.google.com/calendar/api/v3/reference/events#start.date
https://developers.google.com/apps-script/reference/calendar/calendar-event#getStartTime()

[jonas0b1011001]

The documentation mentions start.date but I think that's just for recurring events

start.date is used for all-day events and should be considered aswell.

[non-bin]

thanks, how's that?

[jonas0b1011001]

That will work!

[derekantrican]

Also #292