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.
So sorry. Fixed in 1b10242.
`location` method is called from https://github.com/tomykaira/rspec-core/blob/modularize_NullColorizer/lib/rspec/core/notifications.rb#L250