Add an API for querying what ORM is being used

Extension libraries sometimes need to know what ORM is being used in order to load the right code. We can't rely on defined?(:ActiveRecord) because users tend to leave the defaultrequire 'rails/all'inapplication.rband load their ORM of choice in addition toActiveRecord.

The info is available at Rails.configuration.generators.options[:rails][:orm], but that presents a few problems:

• I'm not certain that there are any guarantees that this will be populated in a situation other than running generators.
• The dependency on generators feels odd for a non-generator activity
• Even with that, it's a long route to traverse to get to high level information

I'd propose adding an API for this like Rails.configuration.orm, or even Rails.orm. I'm happy to submit a patch for this, but would like to get a conversation going first.