Add support for grape version cascading

[qinix]

Support API endpoints like so:

module API::V1
  class Root < Grape::API
    version [:v1, :v2], using: :path

    mount API::V1::Places
    mount API::V1::Users

    add_swagger_documentation api_version: 'v1', mount_path: '/docs'
  end
end

Use the first value in the array as the version component in url.

related issue

• Add support for grape version cascading ruby-grape/grape-swagger#535
• Support for grape version cascading ruby-grape/grape-swagger#146

[LeFnord]

thanks @qinix for open the PR, is it ok for you if I take it over and cherry pick your work to make it mergeable?

[LeFnord]

@qinix … for that I'll close this PR and cherry-pich your changes

[qinix]

@LeFnord 👍