Added ability to change the description and parameters of the API endpoints generated by grape-swagger.

You can now specify additional options for the API endpoints generated by grape-swagger, including the description. In our API we also inject additional options that are processed by middleware (for authentication).

```ruby
add_swagger_documentation \
   api_documentation: { desc: 'First', params: { x: 1 }, xx: 11 },
   specific_api_documentation: { desc: 'Second', params: { y: 42 }, yy: 4242 }
```

I am not in love with the names of these options (open to suggestions).

@CraigCottingham, mind taking a look?