* clean up `smartypants()` so that it uses a pattern matcher form the
   larger trivial replacements it does
 * the particular type of whitespace is not important when matching
   footnote tags ( "this\nis matches "this is", for example.)
 * handle both <orc@pell> and <mailto:orc@pell> automatic links.
 * `mkd_dump()` **USES** the output `FILE*` now.
 * quoted blocks are no longer parsed as toplevel blocks now that
   toplevel blocks have different paragraph handling.
 * `smartypants()` converts `1/4` to `&frac14;`, `1/2` to `&frac12;`, and
   `3/4` to `&frac34;`.   It also knows about 1/4th and 3/4ths.