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)
v Help and version arguments (2021.05.22)
v Application-specific directory for Google secret (2021.05.22)
v Read eid from email on standard input (2021.05.22)

2021.05.22:
v Verbose flag to display attendees and response statuses
v Remove `unwrap`s, implement `todo!`s
v Man page (2021.05.23)
v Add documentation