Only add keys where no orphan records exist

[samjewell]

I love using this gem - thank you everyone for writing and maintaining it. I have a feature request: Could we [optionally] only generate foreign-key migrations where no foreign keys are present? Then I could run

DATABASE_URL=[production_database_url] rails generate immigration AddKeys omit_when_orphans_present

I would have certainty that I could apply the migration without breaking our main build and release process (and other migrations queued up behind mine, that my team-members are trying to run). And I'd then be able to run:
rails g immigration AddRemainingKeys afterwards to see the keys where I need to address the orphan records.