Other changes:
* The format declarations have been broken down in multiple
parts. This makes it possible to structure the implementation
libraries in a way such that the format name is accessible even
when importing only reader or writer modules.
* Flavors lead to more complicated Reader and Writer types.
Instead of requiring the user to write down long type names
such as `JsonReader[DefaultFlavor]`, it's now possible to refer
to the reader and writer type of each format using expressions
such as `Json.Reader` and `Json.Writer`.
* Fixed a typo (PreferredOutput)
