Improved Markdown table rendering

The Taffrail Advice Builder supports Markdown syntax for writing and formatting Advice text. If you know Markdown, you know Markdown tables are... fun.

When using the HTML Advice Content Type, any converted-to-HTML table always included a table heading element (<thead>) whether or not a table heading was present in the Markdown syntax inside Advice Builder.

This has been improved, so now a <thead> will only be present if there is a table header.