Basic Markdown support

[netvl]

Tracking issue for basic Markdown syntax support.

• Paragraphs
  • [x] Single emphasis
  • [x] Double emphasis
  • [x] Inline code blocks
  • [ ] Newlines
  • Links
  • [x] Inline links
  • [x] Reference links
  • [x] Shortcut reference links
  • [ ] Automatic links
  • [x] Images
• [ ] HTML blocks
• Headers
  • [x] ATX headers
  • [x] Setext headers
  • [x] ATX headers right after paragraphs
• [x] Blockquotes
• [x] Indented code blocks
• [ ] Fenced code blocks
• [x] Horizontal rules
• [ ] Reference links blocks
• Lists
  • [ ] Ordered lists
  • [ ] Unordered lists