Note: A `job` is a table with the following keys:
- cmd: command string
- running: boolean whether the job is running
- id: unique id for the job
- pid: process id for the job
- exitCode: exit code of the job
In ordinary cases you'd prefer to use the id instead of pid. The id is unique to
Hilbish and is how you get jobs with the `hilbish.jobs` interface.

+ `job.start` -> job > Thrown when a new background job starts.

+ `job.done` -> job > Thrown when a background jobs exits.