Add Markdown parser (#11)

[valpackett]

WIP CommonMark parser // currently the block structure stage:

• thematic breaks (hr)
• ATX headings
• Setext headings
• indented code blocks
• fenced code blocks
• HTML blocks
• link reference definitions
• paragraphs
• block quotes
• lists

[ronaldtse]

@valpackett ping for progress?

[valpackett]

Been working on the inline part, the emphasis rules are a bit of a nightmare but I mostly have them working already (so far spec examples 350-415 are passing), also some other stuff like entities/escapes. Emphasis is the hard part, other inline elements should be easier to add..