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/colours.ex

@@ -1,4 +1,5 @@
 defmodule Display.Paint do
+  @moduledoc false
   def red(str), do: painter().red(str)
   def cyan(str), do: painter().cyan(str)
   def green(str), do: painter().green(str)
@@ -13,6 +14,7 @@ defmodule Display.Paint do
 end
 
 defmodule Display.Colours do
+  @moduledoc false
   alias IO.ANSI
 
   def red(str), do: colourize(ANSI.red(), str)
@@ -26,6 +28,7 @@ defmodule Display.Colours do
 end
 
 defmodule Display.Uncoloured do
+  @moduledoc false
   def red(str), do: str
   def cyan(str), do: str
   def green(str), do: str