Refactor to support pluggable examples

[skyzyx]

Being able to install sphinxcontrib-httpexample followed by sphinxcontrib-httpexample-guzzle would be great.

Specifically, if the examples used something easy to use like Jinja2, it would be much easier for people to develop their own plugins for examples.

[datakurre]

Because Python is dynamic language, you could extend the plugin could already with custom Sphinx-plugins, by adding their own example builder into AVAILABLE_BUILDERS dictionary of sphinxcontrib.httpexample.directives.

https://github.com/collective/sphinxcontrib-httpexample/blob/master/src/sphinxcontrib/httpexample/directives.py#L12

Builder is simply a function, which receives Python built-in BaseHTTPRequestHandler with example request data and should return a string of corresponding example syntax for a similar request.

I agree that documentation for extending the plugin would be nice. I keep that in mind, when I'm working on this extension next time.