Modularize NullColorizer

Other colorizers s.t. ConsoleCodes are module with module_functions.
NullColorizer was a class and the class does not respond to `wrap()`. This
patch fixes the mismatch.

I approached a problem about NullColorizer in PR #1637, but the fix was not complete.
I am sorry about my carelessness.
You need to use hash rocket syntax here, as we support `1.8.7` and this breaks the travis build.

Also why is this necessary, it's not clear to me...