Add params option for button_to

The parameters are rendered as hidden form fields within the generated form. This is useful for when a record has multiple buttons associated with it, each of which target the same controller method, but which need to submit different attributes.

For example, a bug tracking system may have buttons to mark an issue as Closed or Fixed. These should both make a PATCH request to `/issues/:id`. This could be implemented by adding member routes to the Issue resource, but for some apps that may result in a large number of additional entries in routes.rb.

I understand that there is already a mechanism for adding additional params to the query string for `button_on`, but it's usually preferable to have this data in the request body.