Enable sprig seed file generation (yaml only at this point) from database records (addresses #3)
Allow users to specify environment and classes when calling Sprig::Harvest.reap (addresses #3)

- Created a Sprig::Harvest class-level configuration
- `Sprig::Harvest.reap` takes an options hash and sets environment (`:env` key) and classes
  (`:classes` key) on Sprig::Harvest's configuration
Extract TsortableHash into its own file due to re-use in multiple places