Create a `spring_booted` ActiveSupport load hook

[andrewn617]

I want to be able to configure spring's file watcher to watch additional files when booting my app. Specifically, I want to update Rails to watch the config/initializers directory of engines defined within the application, so that spring will automatically reload when updating those initializers like it does with the root config/initializers.

I define an ActiveSupport load hook called spring_booted. This can be hooked into in an initializer in an app to do additional configuration.