Improved rendering; comments in Markdown

• You can now use HTML comments in your Markdown files --- md4c-reader
  and pango-markup cooperate to remove those comments from the output (#28)
• pango-markup: Code blocks within bulleted or numbered lists are indented
• pango-markup: Consecutive headers have whitespace between them (#29)

Additionally:

• Added a changelog!
• Special blocks are now supported in core (#7)
• md4c-reader: special blocks can themselves include Markdown (#23)