Added api_file_name to config

In our environment, we unfortunately couldn't following the Swagger specs to the letter. The resources file needed to be named 'swagger-docs.json'. This change allows the specification of the file name. For example, in a Rails initializer:

```
Swagger::Docs::Config.register_apis({
  "v1.0" => {
    api_file_name: "swagger-doc.json"
  }
})
```