Pass configuration to SwaggerUIBundle

It would be nice if it was possible to configure Swagger-UI via a parameter,

For example if you want control which sections are expanded.

```javascript
SwaggerUIBundle({
...
"docExpansion":"none"
});
```

I'm not sure how to do this nicely, but maybe a configuration in `mkdocs.yaml` and in the `extra` section, or maybe it's easier to supply it every time you use the plugin?

```
!!swagger FILENAME "docExpansion":"none","someOtherSetting":"value"!!
```

An easier option would be to use `configUrl`, I believe that SwaggerUIBundle will load from that URL.
```
!!swagger FILENAME SWAGGER_CONFIG!!

!!swagger URL SWAGGER_CONFIG_URL!!
```

Thank you for creating the plugin!

Start with the plugin's SwaggerUIBundle invocation and the MkDocs configuration, including mkdocs.yaml and its extra section. Compare the proposed per-use settings with configUrl, then verify that a setting such as docExpansion reaches Swagger UI through the chosen configuration path.

Written by the indexing model from the issue text.