Microformats 2 Rendering

@snarfed, @pfefferle and I were discussing the problem of various plugin interoperability in https://github.com/pfefferle/SemPress/issues/51

It occurred to me, if we are going to set this up, this repo is probably the better place to keep the discussion and possibly any functionality or such we want to support it.

Right now, Micropub takes over rendering if add_theme_support( 'microformats2' ) is not set. This plugin loads microformats2 compatibility functions(add h-entry, etc) if the same flag is set.

So, the add theme support flag should, in future, be used solely to discuss a theme's support for marking up standard WordPress elements, as opposed to properties stored solely in meta. We can be more granular in future if needed.

Rendering microformats2 properties from Post Meta probably requires a global variable(the same way that add_theme_support, register_post_type, etc are in reality global variables).

The issue that was being discussed was priority on any given feature. WordPress registration does this with the last one to register, as registration is done by hook. Same can apply here.

While I still think we can combine our rendering efforts, for now, I want to try to build this as a class here in Microformats 2, which could then be included in the other plugins. Thoughts?

Start with the linked SemPress discussion and the existing Microformats 2 rendering behavior around add_theme_support, post meta, and WordPress hooks. Clarify the proposed class design, registration priority, and interoperability expectations before identifying the implementation scope; done means the project has an agreed rendering approach and corresponding functionality.

Written by the indexing model from the issue text.