Add !hire command

[zpatrick]

Add a new !hire command that will manage candidates along the hiring pipeline.

We should support the following operations:
!hire add: Adds a candidate to the hiring pipeline. While in the hiring pipeline, the candidate's manager will get reminded every day to complete their pipeline until all steps in the pipeline have been completed.

!hire add CANDIDATE

!hire show: Shows which step of the pipeline a candidate is in. If --all is specified, shows all steps in the candidate's pipeline.

!hire show [--all] CANDIDATE

!hire list: Shows the names of all candidates currently in a hire pipeline.

!hire ls [--count=10] [--ascending]

!hire remove: Removes a candidate from the hiring pipeline

!hire rm CANDIDATE

!hire next/step (name TBD) Marks the current candidates' step in the pipeline as completed.
Moves on to the next step in the pipeline, or completes the pipeline.

!hire next/step CANDIDATE

!hire prev (name TBD) Goes back one step in the candidate's pipeline; the inverse of !candidate next/step

!hire prev CANDIDATE

We could also consider !hire step as subcommand, and have the following structure:

!hire add JohnDoe
!hire step ls JohnDoe
!hire step next JohnDoe
!hire step prev JohnDoe