Add !interview command

[zpatrick]

Add/update the !interview command to work with Candidates.

We should support the following operations:
!interview add: Adds new interview at the specified date and time. Each @interviewee will get a reminder on the day of the interview. TBD when exactly that reminder should be sent, and if there should be pre-post interview reminders.

!interview add CANDIDATE DATE TIME @INTERVIEWEES...

!interview ls: List the current interviews

!interview ls [--count=10]

!interview remove: Removes an interview

!interview rm CANDIDATE DATE TIME