fix tag stripping; add unicode char conversions; allow keeping files

1) Fix tag stripping: font, span and div tags were only being removed
   if they included something after the tag name, e.g. <div style="">
   they were being removed, but if they did not, e.g. <div>, then
   they were not. This is fixed.

2) Add unicode char conversions: Converts wingdings characters to
   the closest available unicode characters. This is NOT all unicode
   characters by any means.

3) Allow keeping files: by adding the --keepfiles option users can
   choose to not delete the /tmp directory with intermediate HTML
   and .wiki files