Add acceptance to RSpec::Rails::DIRECTORY_MAPPINGS

Adding `:acceptance` to `DIRECTORY_MAPPINGS` and reapplying `infer_spec_type_from_file_location!` you can discern acceptance examples using `example.metadata[:type]`.

In your `spec/rails_helper.rb` for example you could put something like this:

```ruby
RSpec.configure do |config|
  config.before(:each) do |example|
    # an acceptance example
    if example.metadata[:type] == :acceptance && example.metadata[:auth]
      # stub authentication here
    end
  end
end
```