Add Traits namespace with CollectionItem

CollectionItem will hold functionality that's specific to items
belonging to a collection. Initially, this covers the retrieval of the
value depending on what's passed into the :text_method, :value_method or
:hint_method params.

These arguments now accept and properly deal with procs

Refs #68

lib/govuk_design_system_formbuilder.rb

@@ -2,6 +2,8 @@
 require 'govuk_design_system_formbuilder/builder'
 require 'govuk_design_system_formbuilder/base'
 
+require 'govuk_design_system_formbuilder/traits/collection_item'
+
 require 'govuk_design_system_formbuilder/elements/hint'
 require 'govuk_design_system_formbuilder/elements/label'
 require 'govuk_design_system_formbuilder/elements/date'

lib/govuk_design_system_formbuilder/traits/collection_item.rb

@@ -0,0 +1,18 @@
+module GOVUKDesignSystemFormBuilder
+  module Elements
+    module Traits
+      module CollectionItem
+      private
+
+        def retrieve(item, text_method)
+          case text_method
+          when Symbol, String
+            item.send(text_method)
+          when Proc
+            text_method.call(item)
+          end
+        end
+      end
+    end
+  end
+end