Serialize mdast to markdown

[bnchi]

fixes #64

TODO handlers :

• [x] paragraph
• [x] text
• [x] strong
• [x] emphasis
• [x] heading
• [x] html
• [x] break
• [x] thematic-break
• [x] code
• [x] blockquote
• • [ ] Complete the tests for blockquote
• [x] image
• [x] inline-code
• [x] link
• [x] list
• [x] list-item
• • [ ] Add the tests for list item
• [x] root
• [x] definition
• [x] image-reference
• [ ] link-reference