Add chalice plugin

[mathewmarcus]

Would you be open to a pull request to add an extension, which would likely resemble the Flask extension, to add support for the AWS Chalice microservice framework?

[sloria]

@mathewmarcus I would rather see chalice supported as a 3rd-party plugin. If you end up implementing this, go ahead and add it to the wiki: https://github.com/marshmallow-code/apispec/wiki/Ecosystem