title: Is it possible to pass arguments to a form in Drupal 8?
body: <p>Picture a YAML routing configuration like this one:</p>
  
  <pre><code>foobar.example:
    path: 'example/{entity_type}/{bundle_name}'
    defaults:
      _form: '\Drupal\foobar\Form\FoobarExampleForm'
      _title: 'Example Form'
      foo: 0
      bar: 1
  </code></pre>
  
  <p>Arguments foo and bar would be passed on to a controller, but how do you pass arguments to forms?  I realize entity_type and bundle_name can be retrieved with arg() or menu_get_object().  Are there any other ways to pass arguments to forms?</p>