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Fix bugs:

  1. properly handle html comments w/o spaces
  2. fix nested list handling with unindented
     list items.
  3. Add config.h to the dependency lists for
     cols & echo, so those functions will properly
     rebuild after a reconfig.
  4. More plan9 fixes from Josh Wood; this one
     adds the flag `_C99_SNPRINTF_EXTENSION` to tell
     the APE cc that, yes, I do really want to be
     using snprintf() and I know that C99 has it
     return the size it wants to populate.
  5. don't describe `<style>` blocks as `mystery node` during `markdown -d`

New features:

  1. add a VALID_DOCUMENT magic flag to the MMIOT
     structure, so that if someone attempts to double-free
     it, the second attempt will just not work instead of
     causing a core dump.
  2. restructure the blocktag[] array into a STRING(), so
     users can add additional block tags as they desire.
  3. in relation to this, add the library function `mkd_add_html5_tags()`,
     which adds (globally, and non-removably) a handful of
     new tags for html5 support.