TODO
====

2021.05.18:
v Figure out how to persist OAuth2 token etc.
v Remove hard-coded event ID (2021.05.21)
v Patch event attendee response status (2021.05.19)

2021.05.21:
x If ID is an eid, get email from eid instead of making an Event.get request (eid contains organiser's email address, not invitee's)
- Help and version arguments
- Application-specific directory for Google secret
v Read eid from email on standard input (2021.05.22)

2021.05.22:
v Verbose flag to display attendees and response statuses
- Remove `unwrap`s, implement `todo!`s
- Man page