Simplify exposed output package methods

[jpreese]

Continuing to work towards more stable and usable package APIs for a conftest v1 release, this PR reworks the output package.

• Remove Manager terminology in favor of types that represent what they output. A package consumer could create an output type by saying output.JSON

• Remove the Flush() and Put() methods on each Outputter in favor of a single Output(). Everywhere in the codebase we were just ranging over the CheckResults collection to Put() them into the output manager and then flushing. This lets us just pass in a collection of results and write them e.g.

outputter := output.Get(format)
if err := outputter.Output(results); err != nil {
  return fmt.Errorf("output results: %w", err)
}

• Replace log.Logger with io.Writer. This gives us, as well as package consumers, way more flexibility in how they can get at the formatted output. Tests can continue to pass inbytes.Buffer, and the Conftest binary can pass in os.Stdout.

[Blokje5]

LGTM! The Output interface is a lot cleaner. It is nice to remove the need for the clients of the package to be aware of the order of calling methods.