Add GET parameter for specifying the query processor

Fix #188.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
I am not sure expressing this as 500 error is needed.. since it implies the server did something wrong, but we clearly *know* what is going on. We could just return an empty list if we don't want to expose the values or simply return all values (which are the ones that *are* available). Btw, this feature was originally used to pre-load placeholder text with examples of values that could be searched for. Not that a service should be necessarily influenced by how it is used, but I think returning any kind of error is not helpful in this case.