Format classes are required to expose a constructor with no arguments,
a getFileExtension() and a sendResponse(opts, callback) method.
The ``opts`` parameter contains:
sink Output stream to send the reponse to
sql SQL query requested by the user
skipfields Comma separate list of fields to skip from output
really only needed with "SELECT *" queries
gn Name of the geometry column (for formats requiring one)
dp Number of decimal points of precision for geometries (if used)
database Name of the database to connect to
user_id Identifier of the user
filename Name to use for attachment disposition
The ``callback`` parameter is a function that is invoked when the
format object finished with sending the result to the sink.
If an error occurs the callback is invoked with an Error argument.
