Add support for registering plugins, ideally through docutils

There's a large community of plugins and documents built around Sphinx, but less so around plain Docutils. That's too bad, because Docutils is a really great piece of software, and the standard toolchain works wonderfully for single-page documents, including academic papers, slide decks, or websites.

As far as I can tell, one of the main missing features is a standardized extension mechanism. Customizable roles and directives are one of Docutils strengths, but if I write a package that defines directives I have a good way to plug it into Sphinx or Pelican, but no natural way to plug it into Docutils directly (one that would allow me to use rst2html5.py directly).

Instead, the most convenient way to do this is to write a custom python driver, as far as I can tell: you need first call register_directive, register_canonical_role, etc; and then you call publish_cmdline. That's a lot of work for end users, compared to Sphinx' list of plugins.

As a result of this, we have many directives and plugins that work with one Docutils frontend but not another. This came up in a recent discussion about bibliography support: there is an excellent extension called sphinxcontrib-bibtex, but it does not work with plain Docutils or with Pelican; only with Sphinx.

It would be great if there was a standard, documented way to extend Docutils. Maybe that could be a setuptools entry point, as is done with Pygments? Ideally one could register extra frontends, backends, directives, and roles that way, and users would just need to list in docutils.conf which plugins they intend to use. I expect many plugins would then be able to target "pure docutils", adding features specific to Sphinx only as a progressive enhancement.

(Oh, and we would likely need a single command line application, instead of one per frontend/backend pair, of course)

Please let me know your thoughts!