linting displays upcoming unreplied-to events ("needsAction")

[mikfreedman]

As a user I need to be able to view upcoming events that aren't replied to
so that I can take action on them in my browser

Precursor to #9

[mikfreedman]

$ calendar-assistant lint 2018-10-01
me@example.com (all times in America/New_York)

2018-10-01  03:30 - 05:00 | Incubate front-end bandwidth (recurring)
                          | attendees: +foo@example.com, ?bar@example.com, -quux@example.com
2018-10-01  07:30 - 08:30 | Cultivate dot-com vortals
                          | attendees: +baz@example.com, +quux@example.com
2018-10-01  07:30 - 08:30 | Monetize next-generation eyeballs (1:1)
                          | attendees: ?foo@example.com