* MKD_NOSTYLE -- treat `<style>` blocks as regular html.
* some github flavored markdown support; gfm_...() input
  methods that put hardbreaks (== two spaces) at the end
  of every input line
* a whole bunch of configure.sh tweaks to work around
  the tiny detail that clang is even more anal-compulsive
  than gcc is.
* support for github flavored markdown backtick-delimited
  code blocks (in addition to tilde-delimited codeblocks)
* in the `markdown` program, add
    1. -S flag (tell markdown to spit out style sections)
    2. -n flag (tell markdown not to output generated text)