cmark 0.24.0

• [API change] Added cmark_node_replace(oldnode, newnode).
• Updated spec.txt to 0.24.
• Fixed edge case with escaped parens in link destination (#97).
  This was also checked against the #82 case with asan.
• Removed unnecessary check for fenced in cmark_render_html.
  It's sufficient to check that the info string is empty.
  Indeed, those who use the API may well create a code block
  with an info string without explicitly setting fenced.
• Updated format of test/smart_punct.txt.
• Updated test/spec.txt, test/smart_punct.txt, and
  spec_tests.py to new format.
• Fixed get_containing_block logic in src/commonmark.c.
  This did not allow for the possibility that a node might have no
  containing block, causing the commonmark renderer to segfault if
  passed an inline node with no block parent.
• Fixed string representations of CUSTOM_BLOCK,
  CUSTOM_INLINE. The old versions raw_inline and
  raw_block were being used, and this led to incorrect xml output.
• Use default opts in python sample wrapper.
• Allow multiline setext header content, as per spec.
• Don't allow spaces in link destinations, even with pointy brackets.
  Conforms to latest change in spec.
• Updated scheme scanner according to spec change. We no longer use
  a whitelist of valid schemes.
• Allow any kind of nodes as children of CUSTOM_BLOCK (#96).
• cmark.h: moved typedefs for iterator into iterator section.
  This just moves some code around so it makes more sense
  to read, and in the man page.
• Fixed make_man_page.py so it includes typedefs again.