cmark 0.18

• Switch to 2-clause BSD license, with agreement of contributors.
• Added Profile build type, make prof target.
• Fixed autolink scanner to conform to the spec. Backslash escapes
  not allowed in autolinks.
• Don't rely on strnlen being available (Nick Wellnhofer).
• Updated scanners for new whitespace definition.
• Added CMARK_OPT_SMART and --smart option, smart.c, smart.h.
• Added test for --smart option.
• Fixed segfault with --normalize (closes #7).
• Moved normalization step from XML renderer to cmark_parser_finish.
• Added options parameter to cmark_parse_document, cmark_parse_file.
• Fixed man renderer's escaping for unicode characters.
• Don't require python3 to make cmark.3 man page.
• Use ASCII escapes for punctuation characters for portability.
• Made options an int rather than a long, for consistency.
• Packed cmark_node struct to fit into 128 bytes.
  This gives a small performance boost and lowers memory usage.
• Repacked delimiter struct to avoid hole.
• Fixed use-after-free bug, which arose when a paragraph containing
  only reference links and blank space was finalized (#9).
  Avoid using parser->current in the loop that creates new
  blocks, since finalize in add_child may have removed
  the current parser (if it contains only reference definitions).
  This isn't a great solution; in the long run we need to rewrite
  to make the logic clearer and to make it harder to make
  mistakes like this one.
• Added 'Asan' build type. make asan will link against ASan; the
  resulting executable will do checks for memory access issues.
  Thanks @JordanMilne for the suggestion.
• Add Makefile target to fuzz with AFL (Nick Wellnhofer)
  The variable $AFL_PATH must point to the directory containing the AFL
  binaries. It can be set as an environment variable or passed to make on
  the command line.