add tests

mix.exs

@@ -31,6 +31,7 @@ defmodule FeedReader.MixProject do
       {:date_time_parser, "~> 1.3.0"},
       {:ex_slop, "~> 0.4", only: :dev},
       {:html_sanitize_ex, "~> 1.5.5"},
+      {:plug, "~> 1.0", only: :test},
       {:req, "~> 0.7.5"},
       {:saxy, "~> 1.6"},
       {:tongue, "~> 2.3.0"}