Use file size from thread JSON

chanarch.py (ChanThread):
  download_files():
    Read filesize from thread JSON and pass it to the Downloader, if
    available. This way, we can skip most files that are already
    downloaded without even connecting to the server.

chanarch.py (Downloader):
  download():
    Allow file size argument, in case the filesize is already known, and
    use it to skip downloading before even connecting to a server if it
    matches the downloaded file's size.
Clean up output

chanarch.py (ChanThread):
  update_info():
    Only state that thread 404'd if debugging
  download_files():
    Only show verbose thread downloading message if debugging

chanarch.py (__main__):
  If verbose, show which thread number is being downloaded, i.e.
  'Downloading thread x/x'