Remove dependency on Turndown

[wheelercj]

Turndown is perfect for converting HTML to markdown when only relatively simple customization is needed, but Stardown has outgrown it. How Stardown converts HTML tables to other formats now almost completely bypasses Turndown, many of the default Turndown rules have been overridden, and #96 would require every Turndown rule to become complicated or duplicated.

Stardown should turn HTML into a syntax tree and work with the syntax tree directly instead of using a framework.