Expose ConfigValue parser

[neko-kai]

It's extremely inconvenient to wrap a ConfigValue into a Config just to be able to pass it to the API:

cv =>
  val dec = derivationDerivedDecoder.value
  val (wrappedValue, path) = cv match {
    case configObject: ConfigObject =>
      configObject.toConfig -> None
    case _ =>
      ConfigValueFactory.fromMap(Map("_root_" -> cv).asJava).toConfig -> Some("_root_")
  }
  path.fold(parser.decode[T](wrappedValue)) {
    parser.decodePath[T](wrappedValue, _)
  }