Add Site#in_source_dir

lib/jekyll/site.rb

@@ -91,6 +91,13 @@ module Jekyll
       end
     end
 
+    # Public: Prefix a given path with the source directory.
+    #
+    # Returns a path which is prefixed with the source directory.
+    def in_source_dir(path)
+      Jekyll.sanitized_path(source, path)
+    end
+
     # The list of collections and their corresponding Jekyll::Collection instances.
     # If config['collections'] is set, a new instance is created for each item in the collection.
     # If config['collections'] is not set, a new hash is returned.