markdown documentation language

This specification is a derivation on markdown. It adds no new features to markdown, but adds additional rules so it is easier to author docs and build tooling around those docs.

This is directly inspired by Electron Docs style-guide and the tooling they've built around it. The grander idea here is formalize the rules already defined by Electron, add more to them to cover more Node.js projects, and then refactor their existing tooling for use with the formalized rule set.

Continued usage by and support for Electron is a primary goal for this project. Node.js is another priority, and is the main reason for this abstraction.