* fix delete_temp_files() deleting in EXEDIR but temp_filename() was
  creating temp files in USERDIR
* make delete_temp_files() public so it can be used by plugins
* add optional `dir` parameter to both delete_temp_files() and
  temp_filename() to allow specifying a different directory, this is
  for example useful when generting markdown previews, the temp file
  should be generated in the project dir in case the readme references
  images that are relative to it, so the web browser can find them.