Update to CommonMark spec 0.24

[robinst]

From the diff between 0.22 and 0.23:

• header -> heading
• horizontal rule -> thematic break
• HtmlTag -> HtmlInline (to follow commonmark.js)
• ATX heading: must be space (check regex)
• Entity or numeric cahracter references in raw HTML
• No more optional whitespace after link label

From the diff between 0.23 and 0.24:

• Update spec example parsing
• Headings with multiple lines
• Parentheses inside the link destination may be escaped
• No spaces in in link destination (even with <>)
• Link scheme whitelist removed (link)

[robinst]

Released 0.4.0 that includes these changes.