added formatting options to swagger metadata

Trying to use the current version of grape-swagger with swagger-ui resulted in
some headaches because by default grape-swagger generates API meta-data in XML
but swagger-ui and the rest of our APIs at my organization are expecting JSON.

To solve this problem I added a section to setup options called `:formatting`.
This hash can accept `:format`, `:default_format`, and
`:default_error_formatter`. These options can be passed in when calling
`add_swagger_documentation`.

Example

```ruby
add_swagger_documentation formatting: {format: :json,
                                       default_format: :json,
                                       default_error_formatter: :json}
```