refactored Markdown class to use traits

[cebe]

all markdown features are now in a trait so composing new markdown
flavors is as simple as adding traits to the class.

Additionally this adds an abstract syntax tree.

This should enable the following things:

• make it easier to get in line with common mark Get in line with CommonMark #62
• allow implementing Add option to disable all HTML #28 easily using the absy and dropping/escaping all html.
• should fix Problem with lists and indented references #53 because the abstract syntax is generated for the complete language before any reference is accessed. Need a test for that
• apply the changes to GithubMarkdown
• apply changes to MarkdownExtra
• fix test breaks
• update docs

[josegonzalez]

+1 :)

[josegonzalez]

Is this necessary?

[cebe]

no, I am not finished with this :)