Improve pure Elm markdown parser

This project is based on the idea Improve pure Elm markdown parser. As the idea described, some specs of the pure Elm markdown parser dillonkearns/elm-markdown are not yet implemented. So, the goal is to complete the current implementation of Github-flavored markdown style, get the automated spec tests passing, and clarify places where it needs to diverge from the spec because of its specific handling of HTML for extensibility.