This PR overhauls the HTML to Markdown conversion functionality in order
to make it more pluggable. This will ultimately allow for supporting a
variety of different HTML input structures (both natively and via
extensions).
As part of this, the `rustdoc_to_markdown` crate has been renamed to
`html_to_markdown`.
The `MarkdownWriter` now accepts a list of trait objects that can be
used to drive the conversion of the HTML into Markdown. Right now we
have some generic handler implementations for going from plain HTML
elements to their Markdown equivalents, as well as some rustdoc-specific
ones.
Release Notes:
- N/A
