Add the Flipper::Middleware::Memoizer middleware

This ensures we make maximum one call per feature per request.

See
https://github.com/jnunemaker/flipper/blob/v0.10.2/docs/Optimization.md#memoizing-middlewareSigned-off-by: Rémy Coutable <remy@rymai.me>

config/initializers/flipper.rb

+require 'flipper/middleware/memoizer'
+
+Rails.application.config.middleware.use Flipper::Middleware::Memoizer,
+  lambda { Feature.flipper }

lib/feature.rb

@@ -39,7 +39,6 @@ class Feature
       get(key).disable
     end
 
-    private
 
     def flipper
       @flipper ||= begin