Flags parses the flag from the command line, setting only user-supplied values on the flagext.Registerer passed to Defaults().

JSON returns a Source that opens the supplied `.json` file and loads it.

Parse is a higher level wrapper for Unmarshal that automatically parses flags and a .yaml file.

Unmarshal merges the values of the various configuration sources and sets them on `dst`.

YAML returns a Source that opens the supplied `.yaml` file and loads it.

Cloneable is a config which can be cloned into a flagext.Registerer Contract: the cloned value must not mutate the original.

Source is a generic configuration source.