Add support for generating examples with Guzzle 6 (PHP)

[skyzyx]

http://docs.guzzlephp.org/en/latest/

[stevepiercy]

@skyzyx custom builders are now possible with the release of 2.0. There's a nice example to follow at https://sphinxcontrib-httpexample.readthedocs.io/en/latest/custom.html. We'd gratefully accept a pull request to add this feature.