The way I see it, Markdown is a way to add semantic formatting to a text document. Yes, you can insert other forms of code into a Markdown document, but at a certain point, inventing a new "markdown" syntax for all of the the non-semantic-text code that you want to add isn't really Markdown anymore, and just kind of misses the point. If I want to write semantic text, I'll use Markdown, if I want to create a PostgreSQL table, I'll use Postgres, etc, etc. I don't see the point of trying to make Markdown an interface for things that is wasn't designed for.