Add frozen string literal comment to files.

Add frozen string literal comment to files.

diff --git a/plugin.rb b/plugin.rb
index bb0df25..6795bab 100644
--- a/plugin.rb
+++ b/plugin.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # name: discourse-graphviz
 # about: Graphviz
 # version: 0.0.1

GitHub sha: 1aeb3fe8