Add a way to handle parsed markup content before render

[paradox460]

Previously, I had a pre_build extension that would parse the generated html from the markdown parser and output metadata for Table of Contents (ToC) generation. It would be run after the Post and Page extensions, and store its own data in its own key. You could then reference it from a template, and render a Table of Contents.

As of v0.18.0, this no longer works. The problem appears to be that the markup parsing occurs after the build steps, but before the write steps. However, one cannot move the ToC generation, as it is, to pre_write, as the ToC metadata is required to render the templates used by the pre_write step.