How to start a background task in Ansible 2.x through the API?

Before Ansible 2, one could run a background module like so:

self.runner = ansible.runner.Runner(
module_name=self.module_name,
module_args=self.module_args,
pattern=self.pattern,
forks=self.forks,
sudo=True,
background=self.background)

How can I get the same background functionality in Ansible 2? I’ve looked at all of the possible Options params and can’t seem to find it.

In the example at http://docs.ansible.com/ansible/dev_guide/developing_api.html#python-api-2-0

You would use async in building your tasks defined in the play_source dict. Effectively in 2.x you are building a play, and that play is where you put those options.