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.

test/support/locales/de.yml

@@ -0,0 +1,8 @@
+de:
+  test_class:
+    author_info:
+      full: Autor %{author} ist vom %{location}
+    has_invalid_argument: Willkommen in %{no_matching_method}
+    hello_world: Hallo Welt
+  nonexistant_test_class:
+    never_called: Nie angerufen

test/support/locales/en.yml

@@ -0,0 +1,8 @@
+en:
+  test_class:
+    author_info:
+      full: Author %{author} is from %{location}
+    has_invalid_argument: Welcome to %{no_matching_method}
+    hello_world: Hello world
+  nonexistant_test_class:
+    never_called: Never called