Allow for manual requiring `Ejectable` module in `ActiveRecord`

[maxveldink]

Currently, when you add activerecord-ejection_seat to your Gemfile, it will auto-require the Ejectable module in ActiveRecord::Base to make it available to all models automatically. There probably should be an option for auto-requiring (current behavior) or manually requiring. This would allow a company to have an explicit extend Ejectable in their ApplicationRecord or only extend Ejectable on the models they want to have that functionality for.