Configuration: Add #configure syntax sugar

This allows us to do

```
Alchemy.configure do |config|
  config.user_class = "Spree::User"
end
```

Author: mamhoff

lib/alchemy.rb

@@ -21,10 +21,7 @@ class << self
     def config
       @_config ||= Alchemy::Configurations::Main.new
     end
-
-    def configure(&blk)
-      yield config
-    end
+    delegate :configure, to: :config
 
     enable_searchable_deprecation_msg = "Use `Alchemy.config.show_page_searchable_checkbox` instead."
     def enable_searchable = config.show_page_searchable_checkbox

lib/alchemy/configuration.rb

@@ -39,6 +39,7 @@ def set(configuration_hash)
 
     alias_method :get, :send
     alias_method :[], :get
+    alias_method :configure, :tap
 
     def show = self
 

spec/libraries/alchemy/configuration_spec.rb

@@ -50,6 +50,15 @@
         expect(configuration.fetch("auto_logout_time", 20)).to eq(40)
       end
     end
+
+    describe "#configure" do
+      it "yields self" do
+        configuration.configure do |conf|
+          conf.auto_logout_time = 15
+        end
+        expect(configuration.auto_logout_time).to eq(15)
+      end
+    end
   end
 
   context "setting with the wrong type" do