Add ability to transform_path for the core API config.

The `path` element of the swagger DSL `{"apis": [{"path": ...}]}`
allows a distinction between the API docs server and the hosted API
location (see https://github.com/wordnik/swagger-core/wiki/No-server-Integrations ).
This adds the ability to provide a custom transform function to swagger-docs which
can arbitrarily modify the currently-autogenerated `path` variable.