Log formatter

Fixes #3

Let me know anyway I can improve! I still feel like I'm learning 'the functional way' so any feedback is appreciated.


We can pattern mage in the function head:
```elixir
defp compile_code(key) when is_atom(key) and key in @valid_patterns, do: nil
defp compile_code(key) do
  raise ArgumentError, message: "$#{key} is an invalid format pattern."
end
```