Introduce definition file path configuration

Fixes #165 and closes #166

Currently the only way to customize the factory definition file
paths is to do it in an initializer right after the
"factory_bot.set_factory_paths" initializer. With this commit,
we can customize factory definition file paths by
setting `config.factory_bot.definition_file_paths`
in config/application.rb or the appropriate environment file.

Once we update the documentation to include this new configuration, we
should be able to close #149 and #180 as well, since using this
configuration can replace the initializer solution for sharing factories
in an engine (the initializer solution will still work, but with the new
configuration you don't need to know anything about the
fbr railtie).

This will also allow us to close #192, since we can use this
configuration with an empty array to disable automatic loading
of factories in development.

features/load_definitions.feature

@@ -1,4 +1,4 @@
-Feature: automatically load step definitions
+Feature: automatically load factory definitions
 
   Background:
     When I successfully run `bundle exec rails new testapp`
@@ -53,7 +53,34 @@ Feature: automatically load step definitions
     When I run `bundle exec rake test` with a clean environment
     Then the output should contain "1 assertions, 0 failures, 0 errors"
 
-  Scenario: use factories advertised by railties/engines/3rd-party gems
+  Scenario: use custom definition file paths
+    When I configure the factories as:
+      """
+      config.factory_bot.definition_file_paths = ["custom_factories_path"]
+      """
+    When I write to "custom_factories_path.rb" with:
+      """
+      FactoryBot.define do
+        factory :user do
+          name { "Frank" }
+        end
+      end
+      """
+    When I write to "test/unit/user_test.rb" with:
+      """
+      require 'test_helper'
+
+      class UserTest < ActiveSupport::TestCase
+        test "use factory" do
+          user = FactoryBot.create(:user)
+          assert_equal 'Frank', user.name
+        end
+      end
+      """
+    When I run `bundle exec rake test` with a clean environment
+    Then the output should contain "1 assertions, 0 failures, 0 errors"
+
+  Scenario: use 3rd-party factories with configured definition file paths
     When I append to "config/application.rb" with:
       """
         require File.expand_path('../../lib/some_railtie/railties.rb', __FILE__)
@@ -62,7 +89,41 @@ Feature: automatically load step definitions
       """
       module SomeRailtie
         class Railtie < ::Rails::Engine
+          config.factory_bot.definition_file_paths << File.expand_path('../factories', __FILE__)
+        end
+      end
+      """
+    When I write to "lib/some_railtie/factories.rb" with:
+      """
+      FactoryBot.define do
+        factory :factory_from_some_railtie, class: 'User' do
+          name { 'Artem' }
+        end
+      end
+      """
+    When I write to "test/unit/user_test.rb" with:
+      """
+      require 'test_helper'
 
+      class UserTest < ActiveSupport::TestCase
+        test "use factory of some_railtie" do
+          user = FactoryBot.create(:factory_from_some_railtie)
+          assert_equal 'Artem', user.name
+        end
+      end
+      """
+    When I run `bundle exec rake test` with a clean environment
+    Then the output should contain "1 assertions, 0 failures, 0 errors"
+
+  Scenario: use 3rd-party factories with an initializer
+    When I append to "config/application.rb" with:
+      """
+        require File.expand_path('../../lib/some_railtie/railties.rb', __FILE__)
+      """
+    When I write to "lib/some_railtie/railties.rb" with:
+      """
+      module SomeRailtie
+        class Railtie < ::Rails::Engine
           initializer "some_railtie.factories", :after => "factory_bot.set_factory_paths" do
             FactoryBot.definition_file_paths << File.expand_path('../factories', __FILE__)
           end

lib/factory_bot_rails/railtie.rb

@@ -7,17 +7,15 @@ require "rails"
 
 module FactoryBotRails
   class Railtie < Rails::Railtie
+    config.factory_bot = ActiveSupport::OrderedOptions.new
+    config.factory_bot.definition_file_paths = FactoryBot.definition_file_paths
 
     initializer "factory_bot.set_fixture_replacement" do
       Generator.new(config).run
     end
 
     initializer "factory_bot.set_factory_paths" do
-      FactoryBot.definition_file_paths = [
-        Rails.root.join("factories"),
-        Rails.root.join("test", "factories"),
-        Rails.root.join("spec", "factories"),
-      ]
+      FactoryBot.definition_file_paths = definition_file_paths
     end
 
     initializer "factory_bot.register_reloader" do |app|
@@ -27,5 +25,13 @@ module FactoryBotRails
     config.after_initialize do
       FactoryBot.find_definitions
     end
+
+    private
+
+    def definition_file_paths
+      config.factory_bot.definition_file_paths.map do |path|
+        Rails.root.join(path)
+      end
+    end
   end
 end