Add credo to the project and:

- Run mix credo --all to identify possible code optimizations
 - Resolve most of the errors generated by credo such as:
   - Numbers larger than 9999 should be written with underscores: 58_127
   - Modules should have a @moduledoc tag
   - Comparison will always return true

lib/display/intro.ex

@@ -1,11 +1,12 @@
 defmodule Display.Intro do
+  @moduledoc false
   alias Display.Paint
 
   def intro(module, modules) do
-    if not (module in modules) do
-      show_intro(module.intro)
-    else
+    if module in modules do
       ""
+    else
+      show_intro(module.intro)
     end
   end