There is no option to add server URL

Currently server URL is static and is set to ‘/’ . User do not have any option to set this to custom value. 

e.g. user should be able to add it like this:

```
SERVER_URL = "http://localhost:8080"
generator.generate_swagger(app, 
                   destination_path=API_URL, 
                   server_url = SERVER_URL)
```
