Make filetype infer opt-in

(Re-issue of https://github.com/rspec/rspec-rails/pull/967 against master)

Begins to address #662 

* Configuration is a bang method rather than a setting, because we don't have a public API for "un-including" modules.
* As a result there is no way to make this "opt-out" without requiring a breaking change to config.
* Newly generated `spec_helper.rb` will opt-in, with an explanatory comment.

**DO NOT MERGE:** Needs a 2.99 deprecation. Needs to be based against master after `ammeter` branch is merged.

@alindeman @myronmarston