Check In to Imbibe Event

[zeitlerc]

As an imbiber, I would like to "check in" to an event so others know that I'm planning to go.
Acceptance Criteria:
Imbiber can "check in" to an event, or say that they are going
Imbiber can "check out" of an event if they are checked in, in case they can no longer go
Imbiber can see who is currently checked in
List of check ins should clear after the designated event time has passed, so check ins do not automatically move to the next event

[zeitlerc]

My general implementation idea is something like "/imbibe checkin" would add the author to a list of attendees.
"/imbibe checkout" would take the attendee off the list.
"/imbibe attendees" would display the list of attendees. Optionally, the attendees list could be added at the end of the existing "/imbibe" command