Add frozen string literal comment to files.

Add frozen string literal comment to files.

diff --git a/plugin.rb b/plugin.rb
index 581a12c..8749c16 100644
--- a/plugin.rb
+++ b/plugin.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # name: discourse-footnote
 # about: Adds markdown.it footnote support to Discourse
 # version: 0.1
diff --git a/spec/pretty_text_spec.rb b/spec/pretty_text_spec.rb
index 8bb2de7..fea49cd 100644
--- a/spec/pretty_text_spec.rb
+++ b/spec/pretty_text_spec.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'rails_helper'
 
 describe PrettyText do

GitHub sha: 9ddce620