Allow for overriding only specific Mustache templates/partials. (#1719)

* Allow for overriding only specific templates/partials. Resolves  #1450.

lib/gollum/views/template_cascade.rb

@@ -0,0 +1,36 @@
+module Precious
+  module Views
+    module TemplateCascade
+      def template_priority_path
+        @@template_priority_path
+      end
+
+      def template_priority_path=(path)
+        @@template_priority_path = File.expand_path(path)
+        @template = nil
+      end
+
+      def first_path_available(name)
+        priority = File.join(template_priority_path, "#{name}.#{template_extension}")
+        default = File.join(template_path, "#{name}.#{template_extension}")
+        File.exists?(priority) ? priority : default
+      end
+
+      # Method should track lib/mustache/settings.rb from Mustache project.
+      def template_file
+        @template_file || first_path_available(template_name)
+      end
+
+      # Method should track lib/mustache.rb from Mustache project.
+      def partial(name)
+        path = first_path_available(name)
+        begin
+          File.read(path)
+        rescue
+          raise if raise_on_context_miss?
+          ""
+        end
+      end
+    end
+  end
+end