jsonalchemy: original tags preserved inside JSON

* When using creator rules that involves regex like `245..` the original
  marc tag is preserved inside the `__meta_metadata__` key for posterior
  needs.

* Enhance the `json_for_marc` producer to use the input tag if the
  producer is defined to do so (See `json_for_marc` documentation).
  (Addresses #1722)

Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>