Set exit code when difference is found (compare mode)

bin/brakeman

@@ -55,6 +55,9 @@ end
 if options[:previous_results_json]
   vulns = Brakeman.compare options.merge(:quiet => options[:quiet])
   puts JSON.pretty_generate(vulns)
+  if options[:exit_on_warn] and (vulns[:new].count + vulns[:fixed].count > 0)
+    exit Brakeman::Warnings_Found_Exit_Code
+  end
 else
   #Run scan and output a report
   tracker = Brakeman.run options.merge(:print_report => true, :quiet => options[:quiet])