config: make danbooru_local_config.rb optional.

Make it so that if danbooru_local_config.rb doesn't exist, we continue
with the default config instead of failing.

config/application.rb

@@ -14,10 +14,22 @@ require "rails/test_unit/railtie"
 
 Bundler.require(*Rails.groups)
 
-require_relative "danbooru_default_config"
-require_relative "danbooru_local_config"
+begin
+  require_relative "danbooru_default_config"
+  require_relative "danbooru_local_config"
+rescue LoadError
+end
 
 module Danbooru
+  mattr_accessor :config
+
+  # if danbooru_local_config exists then use it as the config, otherwise use danbooru_default_config.
+  if defined?(CustomConfiguration)
+    self.config = EnvironmentConfiguration.new(CustomConfiguration.new)
+  else
+    self.config = EnvironmentConfiguration.new(Configuration.new)
+  end
+
   class Application < Rails::Application
     # Use the responders controller from the responders gem
     config.app_generators.scaffold_controller :responders_controller