Support a default controller

[zocky]

AFAICT, a controller has to be specified via x-swagger-router-controller for each path, which can be overriden for each operation in the path.

It would be great if a default controller could be defined that would apply to any paths and operations that don't have a specific controller specified, e.g. by including x-swagger-router-controller either directly in the top level of the specification file, or under paths.