Adding yaml as option for generator

New installations will get :yaml as it's default with openapi 3 as the
version. Old installations will have the key missing and will default
to :json with an easy upgrade path.

rswag-specs/lib/rswag/specs.rb

@@ -12,6 +12,7 @@ module Rswag
       c.add_setting :swagger_root
       c.add_setting :swagger_docs
       c.add_setting :swagger_dry_run
+      c.add_setting :swagger_format
       c.extend ExampleGroupHelpers, type: :request
       c.include ExampleHelpers, type: :request
     end