Add I18n interface for mustache templates

This commit adds an interface that allows mustache templates to get I18n
translation strings, transform any arguments that may be present in
them, and then render them on the frontend.

This is our first real step to getting internationalizing the Gollum
frontend.

lib/gollum/app.rb

@@ -15,6 +15,7 @@ require 'pathname'
 require 'gollum'
 require 'gollum/assets'
 require 'gollum/views/helpers'
+require 'gollum/views/helpers/locale_helpers'
 require 'gollum/views/layout'
 require 'gollum/views/editable'
 require 'gollum/views/has_page'

lib/gollum/views/helpers/locale_helpers.rb

@@ -0,0 +1,82 @@
+module Precious
+  module Views
+    module LocaleHelpers
+      NO_METHOD_MESSAGE = 'Argument must be a view method'
+      YAML_VARIABLE_REGEXP = /\%\{[\w]+\}/
+
+      # Returns all I18n translation strings for the current view class.
+      # This method support YAML arguments. For example:
+      #
+      #     last_edited: This content was last edited at %{date}.
+      #
+      # Where the `date` argument must be a method available on the current
+      # class.
+      #
+      # Use this interface within Mustache templates to render any user
+      # interface strings in the current locale. For example:
+      #
+      #     {{ t.last_edited }}
+      #
+      def t
+        autofill I18n.t(locale_klass_name)
+      end
+
+      private
+
+      # Recursively looks up I18n translation values and autofills any YAML
+      # arguments with the return value of the current class's matching method.
+      #
+      # When a translation value with an argument has no matching method, we
+      # then return that value transformed to include the `no_method_message`
+      #
+      def autofill(yaml)
+        yaml.map { |i18n_key, i18n_value|
+          if i18n_value.is_a? Hash
+            [i18n_key, autofill(i18n_value)]
+          elsif has_arguments?(i18n_value)
+            fill_argument_content(i18n_key, i18n_value)
+          else
+            [i18n_key, i18n_value]
+          end
+        }.to_h
+      end
+
+      def fill_argument_content(i18n_key, i18n_value)
+        i18n_value.gsub!(YAML_VARIABLE_REGEXP) do |argument|
+          method_name = argument.gsub(/[^\w]/, '')
+
+          next if method_name.nil?
+
+          begin
+            self.public_send(method_name)
+          rescue NoMethodError => error
+            no_method_message(method_name)
+          end
+        end
+
+        [i18n_key, i18n_value]
+      end
+
+      def has_arguments?(i18n_value)
+        i18n_value.match?(YAML_VARIABLE_REGEXP)
+      end
+
+      # Returns the current class name in a format that is acceptable in YAML.
+      # To summarize its function:
+      #
+      #     NameOfConstant => name_of_constant
+      #
+      def locale_klass_name
+        @locale_klass_name ||= self.class.name.gsub(/::/, '/').
+          gsub(/([A-Z]+)([A-Z][a-z])/,'\1_\2').
+          gsub(/([a-z\d])([A-Z])/,'\1_\2').
+          tr('-', '_').
+          downcase
+      end
+
+      def no_method_message(method_name, message = NO_METHOD_MESSAGE)
+        "[#{message}: #{method_name}]"
+      end
+    end
+  end
+end

lib/gollum/views/layout.rb

@@ -6,10 +6,11 @@ module Precious
       include Rack::Utils
       include Sprockets::Helpers
       include Precious::Views::AppHelpers
+      include Precious::Views::LocaleHelpers
       include Precious::Views::SprocketsHelpers
       include Precious::Views::RouteHelpers
       include Precious::Views::OcticonHelpers
-      
+
       alias_method :h, :escape_html
 
       attr_reader :name, :path