Fix regression that prevents custom named_formats

Version 2.1 allowed users to override or even create their own custom named formats.

This ability was removed in v3.0 in d301625.

Based on the commit message and lack of changes in the Readme, I do not think that this was intentional. 

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* Update the Readme to reflect that changes made in d301625 that extracted `.named_formats` to the `Formatter` module
* Reimplement the ability to change or create custom named formats
* Add tests to document the ability that named formats can be changed or created