Add interruption hook

[Mangara]

A first stab at addressing #103

This would add a hook to let jobs define their own interruption behaviour:

class JobThatInterrupts < ActiveJob::Base
    include JobIteration::Iteration

    stop_iterating_on(->(job) { !Database.healthy? })
    stop_iterating_on(->(job) { MyThrottle.throttle? })

    ...
  end