Support dependency injection for routes engine

Allow a user to define which Rails engine to define routes on when using
multiple engines within a Rails application. This will ensure that path
helpers will correctly be available in the desired engine. This will
default to the default main Rails application if another engine is not
specified.

Resolves issue [#138](#138)

config/routes.rb

@@ -1,4 +1,4 @@
-Rails.application.routes.draw do
+HighVoltage.parent_engine.routes.draw do
   if HighVoltage.home_page
     get "/#{HighVoltage.home_page}", to: redirect('/')
     root to: 'high_voltage/pages#show', id: HighVoltage.home_page

lib/high_voltage/configuration.rb

@@ -9,6 +9,7 @@ module Configuration
       :content_path,
       :home_page,
       :layout,
+      :parent_engine,
       :route_drawer,
       :routes,
     )

lib/high_voltage/engine.rb

@@ -1,5 +1,9 @@
 module HighVoltage
   class Engine < Rails::Engine
+    initializer 'Set up default parent engine' do |app|
+      HighVoltage.parent_engine ||= Rails.application
+    end
+
     initializer 'Require concerns path' do |app|
       concerns_path = 'app/controllers/concerns'
 

spec/high_voltage/configuration_spec.rb

@@ -12,17 +12,25 @@
           config.content_path = config_value
           config.layout = config_value
           config.page_caching = config_value
+          config.parent_engine = config_value
           config.route_drawer = config_value
           config.routes = config_value
         end
       end
     end
 
+    after(:each) do
+      HighVoltage.configure do |config|
+        config.parent_engine = Rails.application
+      end
+    end
+
     it { expect(HighVoltage.action_caching).to eq config_value }
     it { expect(HighVoltage.action_caching_layout).to eq config_value }
     it { expect(HighVoltage.content_path).to eq config_value }
     it { expect(HighVoltage.layout).to eq config_value }
     it { expect(HighVoltage.page_caching).to eq config_value }
+    it { expect(HighVoltage.parent_engine).to eq config_value }
     it { expect(HighVoltage.route_drawer).to eq config_value }
     it { expect(HighVoltage.routes).to eq config_value }
   end