Make output public.

getOutputStream() and getWhere() already are. So: convenience.
Use the Validator to validate the parameters.

In addition to checking that the paths work, we now also ask
the source config if they're valid for other reasons.
To start, ParameterizedConfig doesn't allow parameterizing its comment.
Progress...

- work on maintaining a singleton instance of a derived config.
  Not yet tested.
- Also mark as invalid any attempt to parameterize the base or the type!
  Not gonna work!
- Some other commented-out cruft.
Avoid frequent string concatination in Validator.

- Just add the 'where' value to the deque, and concatinate the "path"
  only when needed.
- Also avoid string concatination when defining a 'where'. Now there's
  no need to add a separator by hand.
Cleanup.
ParameterizedConfig needs to reject config_type.

That property will be on any config if DerivedConfig is in the mix.