Add plugin factory - plugins specific for each flow

This changes how plugins are defined. Now, instead of defining them as classes in a huge Plugins object, they are defined in /lib/smartdown_plugins/<flow_name>.rb

See https://github.com/alphagov/smart-answers/blob/2433e10524d0c6324772d8ddfbd0dc19f1d31e8f/lib/smartdown_plugins/animal-example-multiple.rb for an example.